Issue mcp-graveyard/UXP#303 Part 1: Move basilisk files from /browser to /application/basilisk

33 files changed, 11386 insertions, 0 deletions

diff --git a/application/basilisk/components/sessionstore/ContentRestore.jsm b/application/basilisk/components/sessionstore/ContentRestore.jsm
new file mode 100644
index 0000000000..d4972bcafe
--- /dev/null
+++ b/application/basilisk/components/sessionstore/ContentRestore.jsm
@@ -0,0 +1,434 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+* License, v. 2.0. If a copy of the MPL was not distributed with this file,
+* You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+"use strict";
+
+this.EXPORTED_SYMBOLS = ["ContentRestore"];
+
+const Cu = Components.utils;
+const Ci = Components.interfaces;
+
+Cu.import("resource://gre/modules/XPCOMUtils.jsm", this);
+
+XPCOMUtils.defineLazyModuleGetter(this, "DocShellCapabilities",
+  "resource:///modules/sessionstore/DocShellCapabilities.jsm");
+XPCOMUtils.defineLazyModuleGetter(this, "FormData",
+  "resource://gre/modules/FormData.jsm");
+XPCOMUtils.defineLazyModuleGetter(this, "PageStyle",
+  "resource:///modules/sessionstore/PageStyle.jsm");
+XPCOMUtils.defineLazyModuleGetter(this, "ScrollPosition",
+  "resource://gre/modules/ScrollPosition.jsm");
+XPCOMUtils.defineLazyModuleGetter(this, "SessionHistory",
+  "resource:///modules/sessionstore/SessionHistory.jsm");
+XPCOMUtils.defineLazyModuleGetter(this, "SessionStorage",
+  "resource:///modules/sessionstore/SessionStorage.jsm");
+XPCOMUtils.defineLazyModuleGetter(this, "Utils",
+  "resource://gre/modules/sessionstore/Utils.jsm");
+
+/**
+ * This module implements the content side of session restoration. The chrome
+ * side is handled by SessionStore.jsm. The functions in this module are called
+ * by content-sessionStore.js based on messages received from SessionStore.jsm
+ * (or, in one case, based on a "load" event). Each tab has its own
+ * ContentRestore instance, constructed by content-sessionStore.js.
+ *
+ * In a typical restore, content-sessionStore.js will call the following based
+ * on messages and events it receives:
+ *
+ *   restoreHistory(tabData, loadArguments, callbacks)
+ *     Restores the tab's history and session cookies.
+ *   restoreTabContent(loadArguments, finishCallback)
+ *     Starts loading the data for the current page to restore.
+ *   restoreDocument()
+ *     Restore form and scroll data.
+ *
+ * When the page has been loaded from the network, we call finishCallback. It
+ * should send a message to SessionStore.jsm, which may cause other tabs to be
+ * restored.
+ *
+ * When the page has finished loading, a "load" event will trigger in
+ * content-sessionStore.js, which will call restoreDocument. At that point,
+ * form data is restored and the restore is complete.
+ *
+ * At any time, SessionStore.jsm can cancel the ongoing restore by sending a
+ * reset message, which causes resetRestore to be called. At that point it's
+ * legal to begin another restore.
+ */
+function ContentRestore(chromeGlobal) {
+  let internal = new ContentRestoreInternal(chromeGlobal);
+  let external = {};
+
+  let EXPORTED_METHODS = ["restoreHistory",
+                          "restoreTabContent",
+                          "restoreDocument",
+                          "resetRestore"
+                         ];
+
+  for (let method of EXPORTED_METHODS) {
+    external[method] = internal[method].bind(internal);
+  }
+
+  return Object.freeze(external);
+}
+
+function ContentRestoreInternal(chromeGlobal) {
+  this.chromeGlobal = chromeGlobal;
+
+  // The following fields are only valid during certain phases of the restore
+  // process.
+
+  // The tabData for the restore. Set in restoreHistory and removed in
+  // restoreTabContent.
+  this._tabData = null;
+
+  // Contains {entry, pageStyle, scrollPositions, formdata}, where entry is a
+  // single entry from the tabData.entries array. Set in
+  // restoreTabContent and removed in restoreDocument.
+  this._restoringDocument = null;
+
+  // This listener is used to detect reloads on restoring tabs. Set in
+  // restoreHistory and removed in restoreTabContent.
+  this._historyListener = null;
+
+  // This listener detects when a pending tab starts loading (when not
+  // initiated by sessionstore) and when a restoring tab has finished loading
+  // data from the network. Set in restoreHistory() and restoreTabContent(),
+  // removed in resetRestore().
+  this._progressListener = null;
+}
+
+/**
+ * The API for the ContentRestore module. Methods listed in EXPORTED_METHODS are
+ * public.
+ */
+ContentRestoreInternal.prototype = {
+
+  get docShell() {
+    return this.chromeGlobal.docShell;
+  },
+
+  /**
+   * Starts the process of restoring a tab. The tabData to be restored is passed
+   * in here and used throughout the restoration. The epoch (which must be
+   * non-zero) is passed through to all the callbacks. If a load in the tab
+   * is started while it is pending, the appropriate callbacks are called.
+   */
+  restoreHistory(tabData, loadArguments, callbacks) {
+    this._tabData = tabData;
+
+    // In case about:blank isn't done yet.
+    let webNavigation = this.docShell.QueryInterface(Ci.nsIWebNavigation);
+    webNavigation.stop(Ci.nsIWebNavigation.STOP_ALL);
+
+    // Make sure currentURI is set so that switch-to-tab works before the tab is
+    // restored. We'll reset this to about:blank when we try to restore the tab
+    // to ensure that docshell doeesn't get confused. Don't bother doing this if
+    // we're restoring immediately due to a process switch. It just causes the
+    // URL bar to be temporarily blank.
+    let activeIndex = tabData.index - 1;
+    let activePageData = tabData.entries[activeIndex] || {};
+    let uri = activePageData.url || null;
+    if (uri && !loadArguments) {
+      webNavigation.setCurrentURI(Utils.makeURI(uri));
+    }
+
+    SessionHistory.restore(this.docShell, tabData);
+
+    // Add a listener to watch for reloads.
+    let listener = new HistoryListener(this.docShell, () => {
+      // On reload, restore tab contents.
+      this.restoreTabContent(null, false, callbacks.onLoadFinished);
+    });
+
+    webNavigation.sessionHistory.addSHistoryListener(listener);
+    this._historyListener = listener;
+
+    // Make sure to reset the capabilities and attributes in case this tab gets
+    // reused.
+    let disallow = new Set(tabData.disallow && tabData.disallow.split(","));
+    DocShellCapabilities.restore(this.docShell, disallow);
+
+    if (tabData.storage && this.docShell instanceof Ci.nsIDocShell) {
+      SessionStorage.restore(this.docShell, tabData.storage);
+      delete tabData.storage;
+    }
+
+    // Add a progress listener to correctly handle browser.loadURI()
+    // calls from foreign code.
+    this._progressListener = new ProgressListener(this.docShell, {
+      onStartRequest: () => {
+        // Some code called browser.loadURI() on a pending tab. It's safe to
+        // assume we don't care about restoring scroll or form data.
+        this._tabData = null;
+
+        // Listen for the tab to finish loading.
+        this.restoreTabContentStarted(callbacks.onLoadFinished);
+
+        // Notify the parent.
+        callbacks.onLoadStarted();
+      }
+    });
+  },
+
+  /**
+   * Start loading the current page. When the data has finished loading from the
+   * network, finishCallback is called. Returns true if the load was successful.
+   */
+  restoreTabContent: function (loadArguments, isRemotenessUpdate, finishCallback) {
+    let tabData = this._tabData;
+    this._tabData = null;
+
+    let webNavigation = this.docShell.QueryInterface(Ci.nsIWebNavigation);
+    let history = webNavigation.sessionHistory;
+
+    // Listen for the tab to finish loading.
+    this.restoreTabContentStarted(finishCallback);
+
+    // Reset the current URI to about:blank. We changed it above for
+    // switch-to-tab, but now it must go back to the correct value before the
+    // load happens. Don't bother doing this if we're restoring immediately
+    // due to a process switch.
+    if (!isRemotenessUpdate) {
+      webNavigation.setCurrentURI(Utils.makeURI("about:blank"));
+    }
+
+    try {
+      if (loadArguments) {
+        // A load has been redirected to a new process so get history into the
+        // same state it was before the load started then trigger the load.
+        let referrer = loadArguments.referrer ?
+                       Utils.makeURI(loadArguments.referrer) : null;
+        let referrerPolicy = ('referrerPolicy' in loadArguments
+            ? loadArguments.referrerPolicy
+            : Ci.nsIHttpChannel.REFERRER_POLICY_DEFAULT);
+        let postData = loadArguments.postData ?
+                       Utils.makeInputStream(loadArguments.postData) : null;
+        let triggeringPrincipal = loadArguments.triggeringPrincipal
+                                  ? Utils.deserializePrincipal(loadArguments.triggeringPrincipal)
+                                  : null;
+
+        if (loadArguments.userContextId) {
+          webNavigation.setOriginAttributesBeforeLoading({ userContextId: loadArguments.userContextId });
+        }
+
+        webNavigation.loadURIWithOptions(loadArguments.uri, loadArguments.flags,
+                                         referrer, referrerPolicy, postData,
+                                         null, null, triggeringPrincipal);
+      } else if (tabData.userTypedValue && tabData.userTypedClear) {
+        // If the user typed a URL into the URL bar and hit enter right before
+        // we crashed, we want to start loading that page again. A non-zero
+        // userTypedClear value means that the load had started.
+        // Load userTypedValue and fix up the URL if it's partial/broken.
+        webNavigation.loadURI(tabData.userTypedValue,
+                              Ci.nsIWebNavigation.LOAD_FLAGS_ALLOW_THIRD_PARTY_FIXUP,
+                              null, null, null);
+      } else if (tabData.entries.length) {
+        // Stash away the data we need for restoreDocument.
+        let activeIndex = tabData.index - 1;
+        this._restoringDocument = {entry: tabData.entries[activeIndex] || {},
+                                   formdata: tabData.formdata || {},
+                                   pageStyle: tabData.pageStyle || {},
+                                   scrollPositions: tabData.scroll || {}};
+
+        // In order to work around certain issues in session history, we need to
+        // force session history to update its internal index and call reload
+        // instead of gotoIndex. See bug 597315.
+        history.reloadCurrentEntry();
+      } else {
+        // If there's nothing to restore, we should still blank the page.
+        webNavigation.loadURI("about:blank",
+                              Ci.nsIWebNavigation.LOAD_FLAGS_BYPASS_HISTORY,
+                              null, null, null);
+      }
+
+      return true;
+    } catch (ex if ex instanceof Ci.nsIException) {
+      // Ignore page load errors, but return false to signal that the load never
+      // happened.
+      return false;
+    }
+  },
+
+  /**
+   * To be called after restoreHistory(). Removes all listeners needed for
+   * pending tabs and makes sure to notify when the tab finished loading.
+   */
+  restoreTabContentStarted(finishCallback) {
+    // The reload listener is no longer needed.
+    this._historyListener.uninstall();
+    this._historyListener = null;
+
+    // Remove the old progress listener.
+    this._progressListener.uninstall();
+
+    // We're about to start a load. This listener will be called when the load
+    // has finished getting everything from the network.
+    this._progressListener = new ProgressListener(this.docShell, {
+      onStopRequest: () => {
+        // Call resetRestore() to reset the state back to normal. The data
+        // needed for restoreDocument() (which hasn't happened yet) will
+        // remain in _restoringDocument.
+        this.resetRestore();
+
+        finishCallback();
+      }
+    });
+  },
+
+  /**
+   * Finish restoring the tab by filling in form data and setting the scroll
+   * position. The restore is complete when this function exits. It should be
+   * called when the "load" event fires for the restoring tab.
+   */
+  restoreDocument: function () {
+    if (!this._restoringDocument) {
+      return;
+    }
+    let {entry, pageStyle, formdata, scrollPositions} = this._restoringDocument;
+    this._restoringDocument = null;
+
+    let window = this.docShell.QueryInterface(Ci.nsIInterfaceRequestor)
+                               .getInterface(Ci.nsIDOMWindow);
+
+    PageStyle.restoreTree(this.docShell, pageStyle);
+    FormData.restoreTree(window, formdata);
+    ScrollPosition.restoreTree(window, scrollPositions);
+  },
+
+  /**
+   * Cancel an ongoing restore. This function can be called any time between
+   * restoreHistory and restoreDocument.
+   *
+   * This function is called externally (if a restore is canceled) and
+   * internally (when the loads for a restore have finished). In the latter
+   * case, it's called before restoreDocument, so it cannot clear
+   * _restoringDocument.
+   */
+  resetRestore: function () {
+    this._tabData = null;
+
+    if (this._historyListener) {
+      this._historyListener.uninstall();
+    }
+    this._historyListener = null;
+
+    if (this._progressListener) {
+      this._progressListener.uninstall();
+    }
+    this._progressListener = null;
+  }
+};
+
+/*
+ * This listener detects when a page being restored is reloaded. It triggers a
+ * callback and cancels the reload. The callback will send a message to
+ * SessionStore.jsm so that it can restore the content immediately.
+ */
+function HistoryListener(docShell, callback) {
+  let webNavigation = docShell.QueryInterface(Ci.nsIWebNavigation);
+  webNavigation.sessionHistory.addSHistoryListener(this);
+
+  this.webNavigation = webNavigation;
+  this.callback = callback;
+}
+HistoryListener.prototype = {
+  QueryInterface: XPCOMUtils.generateQI([
+    Ci.nsISHistoryListener,
+    Ci.nsISupportsWeakReference
+  ]),
+
+  uninstall: function () {
+    let shistory = this.webNavigation.sessionHistory;
+    if (shistory) {
+      shistory.removeSHistoryListener(this);
+    }
+  },
+
+  OnHistoryGoBack: function(backURI) { return true; },
+  OnHistoryGoForward: function(forwardURI) { return true; },
+  OnHistoryGotoIndex: function(index, gotoURI) { return true; },
+  OnHistoryPurge: function(numEntries) { return true; },
+  OnHistoryReplaceEntry: function(index) {},
+
+  // This will be called for a pending tab when loadURI(uri) is called where
+  // the given |uri| only differs in the fragment.
+  OnHistoryNewEntry(newURI) {
+    let currentURI = this.webNavigation.currentURI;
+
+    // Ignore new SHistory entries with the same URI as those do not indicate
+    // a navigation inside a document by changing the #hash part of the URL.
+    // We usually hit this when purging session history for browsers.
+    if (currentURI && (currentURI.spec == newURI.spec)) {
+      return;
+    }
+
+    // Reset the tab's URL to what it's actually showing. Without this loadURI()
+    // would use the current document and change the displayed URL only.
+    this.webNavigation.setCurrentURI(Utils.makeURI("about:blank"));
+
+    // Kick off a new load so that we navigate away from about:blank to the
+    // new URL that was passed to loadURI(). The new load will cause a
+    // STATE_START notification to be sent and the ProgressListener will then
+    // notify the parent and do the rest.
+    let flags = Ci.nsIWebNavigation.LOAD_FLAGS_ALLOW_THIRD_PARTY_FIXUP;
+    this.webNavigation.loadURI(newURI.spec, flags, null, null, null);
+  },
+
+  OnHistoryReload(reloadURI, reloadFlags) {
+    this.callback();
+
+    // Cancel the load.
+    return false;
+  },
+}
+
+/**
+ * This class informs SessionStore.jsm whenever the network requests for a
+ * restoring page have completely finished. We only restore three tabs
+ * simultaneously, so this is the signal for SessionStore.jsm to kick off
+ * another restore (if there are more to do).
+ *
+ * The progress listener is also used to be notified when a load not initiated
+ * by sessionstore starts. Pending tabs will then need to be marked as no
+ * longer pending.
+ */
+function ProgressListener(docShell, callbacks) {
+  let webProgress = docShell.QueryInterface(Ci.nsIInterfaceRequestor)
+                            .getInterface(Ci.nsIWebProgress);
+  webProgress.addProgressListener(this, Ci.nsIWebProgress.NOTIFY_STATE_WINDOW);
+
+  this.webProgress = webProgress;
+  this.callbacks = callbacks;
+}
+
+ProgressListener.prototype = {
+  QueryInterface: XPCOMUtils.generateQI([
+    Ci.nsIWebProgressListener,
+    Ci.nsISupportsWeakReference
+  ]),
+
+  uninstall: function() {
+    this.webProgress.removeProgressListener(this);
+  },
+
+  onStateChange: function(webProgress, request, stateFlags, status) {
+    let {STATE_IS_WINDOW, STATE_STOP, STATE_START} = Ci.nsIWebProgressListener;
+    if (!webProgress.isTopLevel || !(stateFlags & STATE_IS_WINDOW)) {
+      return;
+    }
+
+    if (stateFlags & STATE_START && this.callbacks.onStartRequest) {
+      this.callbacks.onStartRequest();
+    }
+
+    if (stateFlags & STATE_STOP && this.callbacks.onStopRequest) {
+      this.callbacks.onStopRequest();
+    }
+  },
+
+  onLocationChange: function() {},
+  onProgressChange: function() {},
+  onStatusChange: function() {},
+  onSecurityChange: function() {},
+};
diff --git a/application/basilisk/components/sessionstore/DocShellCapabilities.jsm b/application/basilisk/components/sessionstore/DocShellCapabilities.jsm
new file mode 100644
index 0000000000..098aae86f0
--- /dev/null
+++ b/application/basilisk/components/sessionstore/DocShellCapabilities.jsm
@@ -0,0 +1,50 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+* License, v. 2.0. If a copy of the MPL was not distributed with this file,
+* You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+"use strict";
+
+this.EXPORTED_SYMBOLS = ["DocShellCapabilities"];
+
+/**
+ * The external API exported by this module.
+ */
+this.DocShellCapabilities = Object.freeze({
+  collect: function (docShell) {
+    return DocShellCapabilitiesInternal.collect(docShell);
+  },
+
+  restore: function (docShell, disallow) {
+    return DocShellCapabilitiesInternal.restore(docShell, disallow);
+  },
+});
+
+/**
+ * Internal functionality to save and restore the docShell.allow* properties.
+ */
+var DocShellCapabilitiesInternal = {
+  // List of docShell capabilities to (re)store. These are automatically
+  // retrieved from a given docShell if not already collected before.
+  // This is made so they're automatically in sync with all nsIDocShell.allow*
+  // properties.
+  caps: null,
+
+  allCapabilities: function (docShell) {
+    if (!this.caps) {
+      let keys = Object.keys(docShell);
+      this.caps = keys.filter(k => k.startsWith("allow")).map(k => k.slice(5));
+    }
+    return this.caps;
+  },
+
+  collect: function (docShell) {
+    let caps = this.allCapabilities(docShell);
+    return caps.filter(cap => !docShell["allow" + cap]);
+  },
+
+  restore: function (docShell, disallow) {
+    let caps = this.allCapabilities(docShell);
+    for (let cap of caps)
+      docShell["allow" + cap] = !disallow.has(cap);
+  },
+};
diff --git a/application/basilisk/components/sessionstore/FrameTree.jsm b/application/basilisk/components/sessionstore/FrameTree.jsm
new file mode 100644
index 0000000000..e8ed12a8ff
--- /dev/null
+++ b/application/basilisk/components/sessionstore/FrameTree.jsm
@@ -0,0 +1,254 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this file,
+ * You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+"use strict";
+
+this.EXPORTED_SYMBOLS = ["FrameTree"];
+
+const Cu = Components.utils;
+const Ci = Components.interfaces;
+
+Cu.import("resource://gre/modules/XPCOMUtils.jsm", this);
+
+const EXPORTED_METHODS = ["addObserver", "contains", "map", "forEach"];
+
+/**
+ * A FrameTree represents all frames that were reachable when the document
+ * was loaded. We use this information to ignore frames when collecting
+ * sessionstore data as we can't currently restore anything for frames that
+ * have been created dynamically after or at the load event.
+ *
+ * @constructor
+ */
+function FrameTree(chromeGlobal) {
+  let internal = new FrameTreeInternal(chromeGlobal);
+  let external = {};
+
+  for (let method of EXPORTED_METHODS) {
+    external[method] = internal[method].bind(internal);
+  }
+
+  return Object.freeze(external);
+}
+
+/**
+ * The internal frame tree API that the public one points to.
+ *
+ * @constructor
+ */
+function FrameTreeInternal(chromeGlobal) {
+  // A WeakMap that uses frames (DOMWindows) as keys and their initial indices
+  // in their parents' child lists as values. Suppose we have a root frame with
+  // three subframes i.e. a page with three iframes. The WeakMap would have
+  // four entries and look as follows:
+  //
+  // root -> 0
+  // subframe1 -> 0
+  // subframe2 -> 1
+  // subframe3 -> 2
+  //
+  // Should one of the subframes disappear we will stop collecting data for it
+  // as |this._frames.has(frame) == false|. All other subframes will maintain
+  // their initial indices to ensure we can restore frame data appropriately.
+  this._frames = new WeakMap();
+
+  // The Set of observers that will be notified when the frame changes.
+  this._observers = new Set();
+
+  // The chrome global we use to retrieve the current DOMWindow.
+  this._chromeGlobal = chromeGlobal;
+
+  // Register a web progress listener to be notified about new page loads.
+  let docShell = chromeGlobal.docShell;
+  let ifreq = docShell.QueryInterface(Ci.nsIInterfaceRequestor);
+  let webProgress = ifreq.getInterface(Ci.nsIWebProgress);
+  webProgress.addProgressListener(this, Ci.nsIWebProgress.NOTIFY_STATE_DOCUMENT);
+}
+
+FrameTreeInternal.prototype = {
+
+  // Returns the docShell's current global.
+  get content() {
+    return this._chromeGlobal.content;
+  },
+
+  /**
+   * Adds a given observer |obs| to the set of observers that will be notified
+   * when the frame tree is reset (when a new document starts loading) or
+   * recollected (when a document finishes loading).
+   *
+   * @param obs (object)
+   */
+  addObserver: function (obs) {
+    this._observers.add(obs);
+  },
+
+  /**
+   * Notifies all observers that implement the given |method|.
+   *
+   * @param method (string)
+   */
+  notifyObservers: function (method) {
+    for (let obs of this._observers) {
+      if (obs.hasOwnProperty(method)) {
+        obs[method]();
+      }
+    }
+  },
+
+  /**
+   * Checks whether a given |frame| is contained in the collected frame tree.
+   * If it is not, this indicates that we should not collect data for it.
+   *
+   * @param frame (nsIDOMWindow)
+   * @return bool
+   */
+  contains: function (frame) {
+    return this._frames.has(frame);
+  },
+
+  /**
+   * Recursively applies the given function |cb| to the stored frame tree. Use
+   * this method to collect sessionstore data for all reachable frames stored
+   * in the frame tree.
+   *
+   * If a given function |cb| returns a value, it must be an object. It may
+   * however return "null" to indicate that there is no data to be stored for
+   * the given frame.
+   *
+   * The object returned by |cb| cannot have any property named "children" as
+   * that is used to store information about subframes in the tree returned
+   * by |map()| and might be overridden.
+   *
+   * @param cb (function)
+   * @return object
+   */
+  map: function (cb) {
+    let frames = this._frames;
+
+    function walk(frame) {
+      let obj = cb(frame) || {};
+
+      if (frames.has(frame)) {
+        let children = [];
+
+        Array.forEach(frame.frames, subframe => {
+          // Don't collect any data if the frame is not contained in the
+          // initial frame tree. It's a dynamic frame added later.
+          if (!frames.has(subframe)) {
+            return;
+          }
+
+          // Retrieve the frame's original position in its parent's child list.
+          let index = frames.get(subframe);
+
+          // Recursively collect data for the current subframe.
+          let result = walk(subframe, cb);
+          if (result && Object.keys(result).length) {
+            children[index] = result;
+          }
+        });
+
+        if (children.length) {
+          obj.children = children;
+        }
+      }
+
+      return Object.keys(obj).length ? obj : null;
+    }
+
+    return walk(this.content);
+  },
+
+  /**
+   * Applies the given function |cb| to all frames stored in the tree. Use this
+   * method if |map()| doesn't suit your needs and you want more control over
+   * how data is collected.
+   *
+   * @param cb (function)
+   *        This callback receives the current frame as the only argument.
+   */
+  forEach: function (cb) {
+    let frames = this._frames;
+
+    function walk(frame) {
+      cb(frame);
+
+      if (!frames.has(frame)) {
+        return;
+      }
+
+      Array.forEach(frame.frames, subframe => {
+        if (frames.has(subframe)) {
+          cb(subframe);
+        }
+      });
+    }
+
+    walk(this.content);
+  },
+
+  /**
+   * Stores a given |frame| and its children in the frame tree.
+   *
+   * @param frame (nsIDOMWindow)
+   * @param index (int)
+   *        The index in the given frame's parent's child list.
+   */
+  collect: function (frame, index = 0) {
+    // Mark the given frame as contained in the frame tree.
+    this._frames.set(frame, index);
+
+    // Mark the given frame's subframes as contained in the tree.
+    Array.forEach(frame.frames, this.collect, this);
+  },
+
+  /**
+   * @see nsIWebProgressListener.onStateChange
+   *
+   * We want to be notified about:
+   *  - new documents that start loading to clear the current frame tree;
+   *  - completed document loads to recollect reachable frames.
+   */
+  onStateChange: function (webProgress, request, stateFlags, status) {
+    // Ignore state changes for subframes because we're only interested in the
+    // top-document starting or stopping its load. We thus only care about any
+    // changes to the root of the frame tree, not to any of its nodes/leafs.
+    if (!webProgress.isTopLevel || webProgress.DOMWindow != this.content) {
+      return;
+    }
+
+    // onStateChange will be fired when loading the initial about:blank URI for
+    // a browser, which we don't actually care about. This is particularly for
+    // the case of unrestored background tabs, where the content has not yet
+    // been restored: we don't want to accidentally send any updates to the
+    // parent when the about:blank placeholder page has loaded.
+    if (!this._chromeGlobal.docShell.hasLoadedNonBlankURI) {
+      return;
+    }
+
+    if (stateFlags & Ci.nsIWebProgressListener.STATE_START) {
+      // Clear the list of frames until we can recollect it.
+      this._frames = new WeakMap();
+
+      // Notify observers that the frame tree has been reset.
+      this.notifyObservers("onFrameTreeReset");
+    } else if (stateFlags & Ci.nsIWebProgressListener.STATE_STOP) {
+      // The document and its resources have finished loading.
+      this.collect(webProgress.DOMWindow);
+
+      // Notify observers that the frame tree has been reset.
+      this.notifyObservers("onFrameTreeCollected");
+    }
+  },
+
+  // Unused nsIWebProgressListener methods.
+  onLocationChange: function () {},
+  onProgressChange: function () {},
+  onSecurityChange: function () {},
+  onStatusChange: function () {},
+
+  QueryInterface: XPCOMUtils.generateQI([Ci.nsIWebProgressListener,
+                                         Ci.nsISupportsWeakReference])
+};
diff --git a/application/basilisk/components/sessionstore/GlobalState.jsm b/application/basilisk/components/sessionstore/GlobalState.jsm
new file mode 100644
index 0000000000..ac2d7c81b0
--- /dev/null
+++ b/application/basilisk/components/sessionstore/GlobalState.jsm
@@ -0,0 +1,84 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this file,
+ * You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+"use strict";
+
+this.EXPORTED_SYMBOLS = ["GlobalState"];
+
+const EXPORTED_METHODS = ["getState", "clear", "get", "set", "delete", "setFromState"];
+/**
+ * Module that contains global session data.
+ */
+function GlobalState() {
+  let internal = new GlobalStateInternal();
+  let external = {};
+  for (let method of EXPORTED_METHODS) {
+    external[method] = internal[method].bind(internal);
+  }
+  return Object.freeze(external);
+}
+
+function GlobalStateInternal() {
+  // Storage for global state.
+  this.state = {};
+}
+
+GlobalStateInternal.prototype = {
+  /**
+   * Get all value from the global state.
+   */
+  getState: function() {
+    return this.state;
+  },
+
+  /**
+   * Clear all currently stored global state.
+   */
+  clear: function() {
+    this.state = {};
+  },
+
+  /**
+   * Retrieve a value from the global state.
+   *
+   * @param aKey
+   *        A key the value is stored under.
+   * @return The value stored at aKey, or an empty string if no value is set.
+   */
+  get: function(aKey) {
+    return this.state[aKey] || "";
+  },
+
+  /**
+   * Set a global value.
+   *
+   * @param aKey
+   *        A key to store the value under.
+   */
+  set: function(aKey, aStringValue) {
+    this.state[aKey] = aStringValue;
+  },
+
+  /**
+   * Delete a global value.
+   *
+   * @param aKey
+   *        A key to delete the value for.
+   */
+  delete: function(aKey) {
+    delete this.state[aKey];
+  },
+
+  /**
+   * Set the current global state from a state object. Any previous global
+   * state will be removed, even if the new state does not contain a matching
+   * key.
+   *
+   * @param aState
+   *        A state object to extract global state from to be set.
+   */
+  setFromState: function (aState) {
+    this.state = (aState && aState.global) || {};
+  }
+};
diff --git a/application/basilisk/components/sessionstore/PageStyle.jsm b/application/basilisk/components/sessionstore/PageStyle.jsm
new file mode 100644
index 0000000000..0424ef6b10
--- /dev/null
+++ b/application/basilisk/components/sessionstore/PageStyle.jsm
@@ -0,0 +1,100 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+* License, v. 2.0. If a copy of the MPL was not distributed with this file,
+* You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+"use strict";
+
+this.EXPORTED_SYMBOLS = ["PageStyle"];
+
+const Ci = Components.interfaces;
+
+/**
+ * The external API exported by this module.
+ */
+this.PageStyle = Object.freeze({
+  collect: function (docShell, frameTree) {
+    return PageStyleInternal.collect(docShell, frameTree);
+  },
+
+  restoreTree: function (docShell, data) {
+    PageStyleInternal.restoreTree(docShell, data);
+  }
+});
+
+// Signifies that author style level is disabled for the page.
+const NO_STYLE = "_nostyle";
+
+var PageStyleInternal = {
+  /**
+   * Collects the selected style sheet sets for all reachable frames.
+   */
+  collect: function (docShell, frameTree) {
+    let result = frameTree.map(({document: doc}) => {
+      let style;
+
+      if (doc) {
+        // http://dev.w3.org/csswg/cssom/#persisting-the-selected-css-style-sheet-set
+        style = doc.selectedStyleSheetSet || doc.lastStyleSheetSet;
+      }
+
+      return style ? {pageStyle: style} : null;
+    });
+
+    let markupDocumentViewer =
+      docShell.contentViewer;
+
+    if (markupDocumentViewer.authorStyleDisabled) {
+      result = result || {};
+      result.disabled = true;
+    }
+
+    return result && Object.keys(result).length ? result : null;
+  },
+
+  /**
+   * Restores pageStyle data for the current frame hierarchy starting at the
+   * |docShell's| current DOMWindow using the given pageStyle |data|.
+   *
+   * Warning: If the current frame hierarchy doesn't match that of the given
+   * |data| object we will silently discard data for unreachable frames. We may
+   * as well assign page styles to the wrong frames if some were reordered or
+   * removed.
+   *
+   * @param docShell (nsIDocShell)
+   * @param data (object)
+   *        {
+   *          disabled: true, // when true, author styles will be disabled
+   *          pageStyle: "Dusk",
+   *          children: [
+   *            null,
+   *            {pageStyle: "Mozilla", children: [ ... ]}
+   *          ]
+   *        }
+   */
+  restoreTree: function (docShell, data) {
+    let disabled = data.disabled || false;
+    let markupDocumentViewer =
+      docShell.contentViewer;
+    markupDocumentViewer.authorStyleDisabled = disabled;
+
+    function restoreFrame(root, data) {
+      if (data.hasOwnProperty("pageStyle")) {
+        root.document.selectedStyleSheetSet = data.pageStyle;
+      }
+
+      if (!data.hasOwnProperty("children")) {
+        return;
+      }
+
+      let frames = root.frames;
+      data.children.forEach((child, index) => {
+        if (child && index < frames.length) {
+          restoreFrame(frames[index], child);
+        }
+      });
+    }
+
+    let ifreq = docShell.QueryInterface(Ci.nsIInterfaceRequestor);
+    restoreFrame(ifreq.getInterface(Ci.nsIDOMWindow), data);
+  }
+};
diff --git a/application/basilisk/components/sessionstore/PrivacyFilter.jsm b/application/basilisk/components/sessionstore/PrivacyFilter.jsm
new file mode 100644
index 0000000000..88713b402b
--- /dev/null
+++ b/application/basilisk/components/sessionstore/PrivacyFilter.jsm
@@ -0,0 +1,135 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+* License, v. 2.0. If a copy of the MPL was not distributed with this file,
+* You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+"use strict";
+
+this.EXPORTED_SYMBOLS = ["PrivacyFilter"];
+
+const Cu = Components.utils;
+
+Cu.import("resource://gre/modules/XPCOMUtils.jsm", this);
+
+XPCOMUtils.defineLazyModuleGetter(this, "PrivacyLevel",
+  "resource:///modules/sessionstore/PrivacyLevel.jsm");
+
+/**
+ * A module that provides methods to filter various kinds of data collected
+ * from a tab by the current privacy level as set by the user.
+ */
+this.PrivacyFilter = Object.freeze({
+  /**
+   * Filters the given (serialized) session storage |data| according to the
+   * current privacy level and returns a new object containing only data that
+   * we're allowed to store.
+   *
+   * @param data The session storage data as collected from a tab.
+   * @return object
+   */
+  filterSessionStorageData: function (data) {
+    let retval = {};
+
+    for (let host of Object.keys(data)) {
+      if (PrivacyLevel.check(host)) {
+        retval[host] = data[host];
+      }
+    }
+
+    return Object.keys(retval).length ? retval : null;
+  },
+
+  /**
+   * Filters the given (serialized) form |data| according to the current
+   * privacy level and returns a new object containing only data that we're
+   * allowed to store.
+   *
+   * @param data The form data as collected from a tab.
+   * @return object
+   */
+  filterFormData: function (data) {
+    // If the given form data object has an associated URL that we are not
+    // allowed to store data for, bail out. We explicitly discard data for any
+    // children as well even if storing data for those frames would be allowed.
+    if (data.url && !PrivacyLevel.check(data.url)) {
+      return;
+    }
+
+    let retval = {};
+
+    for (let key of Object.keys(data)) {
+      if (key === "children") {
+        let recurse = child => this.filterFormData(child);
+        let children = data.children.map(recurse).filter(child => child);
+
+        if (children.length) {
+          retval.children = children;
+        }
+      // Only copy keys other than "children" if we have a valid URL in
+      // data.url and we thus passed the privacy level check.
+      } else if (data.url) {
+        retval[key] = data[key];
+      }
+    }
+
+    return Object.keys(retval).length ? retval : null;
+  },
+
+  /**
+   * Removes any private windows and tabs from a given browser state object.
+   *
+   * @param browserState (object)
+   *        The browser state for which we remove any private windows and tabs.
+   *        The given object will be modified.
+   */
+  filterPrivateWindowsAndTabs: function (browserState) {
+    // Remove private opened windows.
+    for (let i = browserState.windows.length - 1; i >= 0; i--) {
+      let win = browserState.windows[i];
+
+      if (win.isPrivate) {
+        browserState.windows.splice(i, 1);
+
+        if (browserState.selectedWindow >= i) {
+          browserState.selectedWindow--;
+        }
+      } else {
+        // Remove private tabs from all open non-private windows.
+        this.filterPrivateTabs(win);
+      }
+    }
+
+    // Remove private closed windows.
+    browserState._closedWindows =
+      browserState._closedWindows.filter(win => !win.isPrivate);
+
+    // Remove private tabs from all remaining closed windows.
+    browserState._closedWindows.forEach(win => this.filterPrivateTabs(win));
+  },
+
+  /**
+   * Removes open private tabs from a given window state object.
+   *
+   * @param winState (object)
+   *        The window state for which we remove any private tabs.
+   *        The given object will be modified.
+   */
+  filterPrivateTabs: function (winState) {
+    // Remove open private tabs.
+    for (let i = winState.tabs.length - 1; i >= 0 ; i--) {
+      let tab = winState.tabs[i];
+
+      if (tab.isPrivate) {
+        winState.tabs.splice(i, 1);
+
+        if (winState.selected >= i) {
+          winState.selected--;
+        }
+      }
+    }
+
+    // Note that closed private tabs are only stored for private windows.
+    // There is no need to call this function for private windows as the
+    // whole window state should just be discarded so we explicitly don't
+    // try to remove closed private tabs as an optimization.
+  }
+});
diff --git a/application/basilisk/components/sessionstore/PrivacyLevel.jsm b/application/basilisk/components/sessionstore/PrivacyLevel.jsm
new file mode 100644
index 0000000000..135f1f9593
--- /dev/null
+++ b/application/basilisk/components/sessionstore/PrivacyLevel.jsm
@@ -0,0 +1,64 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+* License, v. 2.0. If a copy of the MPL was not distributed with this file,
+* You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+"use strict";
+
+this.EXPORTED_SYMBOLS = ["PrivacyLevel"];
+
+const Cu = Components.utils;
+
+Cu.import("resource://gre/modules/Services.jsm");
+
+const PREF = "browser.sessionstore.privacy_level";
+
+// The following constants represent the different possible privacy levels that
+// can be set by the user and that we need to consider when collecting text
+// data, and cookies.
+//
+// Collect data from all sites (http and https).
+const PRIVACY_NONE = 0;
+// Collect data from unencrypted sites (http), only.
+const PRIVACY_ENCRYPTED = 1;
+// Collect no data.
+const PRIVACY_FULL = 2;
+
+/**
+ * The external API as exposed by this module.
+ */
+var PrivacyLevel = Object.freeze({
+  /**
+   * Returns whether the current privacy level allows saving data for the given
+   * |url|.
+   *
+   * @param url The URL we want to save data for.
+   * @return bool
+   */
+  check: function (url) {
+    return PrivacyLevel.canSave({ isHttps: url.startsWith("https:") });
+  },
+
+  /**
+   * Checks whether we're allowed to save data for a specific site.
+   *
+   * @param {isHttps: boolean}
+   *        An object that must have one property: 'isHttps'.
+   *        'isHttps' tells whether the site us secure communication (HTTPS).
+   * @return {bool} Whether we can save data for the specified site.
+   */
+  canSave: function ({isHttps}) {
+    let level = Services.prefs.getIntPref(PREF);
+
+    // Never save any data when full privacy is requested.
+    if (level == PRIVACY_FULL) {
+      return false;
+    }
+
+    // Don't save data for encrypted sites when requested.
+    if (isHttps && level == PRIVACY_ENCRYPTED) {
+      return false;
+    }
+
+    return true;
+  }
+});
diff --git a/application/basilisk/components/sessionstore/RecentlyClosedTabsAndWindowsMenuUtils.jsm b/application/basilisk/components/sessionstore/RecentlyClosedTabsAndWindowsMenuUtils.jsm
new file mode 100644
index 0000000000..ac5731160f
--- /dev/null
+++ b/application/basilisk/components/sessionstore/RecentlyClosedTabsAndWindowsMenuUtils.jsm
@@ -0,0 +1,214 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+this.EXPORTED_SYMBOLS = ["RecentlyClosedTabsAndWindowsMenuUtils"];
+
+const kNSXUL = "http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul";
+
+var Ci = Components.interfaces;
+var Cc = Components.classes;
+var Cr = Components.results;
+var Cu = Components.utils;
+
+Cu.import("resource://gre/modules/XPCOMUtils.jsm");
+Cu.import("resource://gre/modules/Services.jsm");
+Cu.import("resource://gre/modules/PlacesUtils.jsm");
+
+XPCOMUtils.defineLazyModuleGetter(this, "PluralForm",
+                                  "resource://gre/modules/PluralForm.jsm");
+XPCOMUtils.defineLazyModuleGetter(this, "SessionStore",
+                                  "resource:///modules/sessionstore/SessionStore.jsm");
+
+var navigatorBundle = Services.strings.createBundle("chrome://browser/locale/browser.properties");
+
+this.RecentlyClosedTabsAndWindowsMenuUtils = {
+
+  /**
+  * Builds up a document fragment of UI items for the recently closed tabs.
+  * @param   aWindow
+  *          The window that the tabs were closed in.
+  * @param   aTagName
+  *          The tag name that will be used when creating the UI items.
+  * @param   aPrefixRestoreAll (defaults to false)
+  *          Whether the 'restore all tabs' item is suffixed or prefixed to the list.
+  *          If suffixed (the default) a separator will be inserted before it.
+  * @param   aRestoreAllLabel (defaults to "menuRestoreAllTabs.label")
+  *          Which localizable string to use for the 'restore all tabs' item.
+  * @returns A document fragment with UI items for each recently closed tab.
+  */
+  getTabsFragment: function(aWindow, aTagName, aPrefixRestoreAll=false,
+                            aRestoreAllLabel="menuRestoreAllTabs.label") {
+    let doc = aWindow.document;
+    let fragment = doc.createDocumentFragment();
+    if (SessionStore.getClosedTabCount(aWindow) != 0) {
+      let closedTabs = SessionStore.getClosedTabData(aWindow, false);
+      for (let i = 0; i < closedTabs.length; i++) {
+        createEntry(aTagName, false, i, closedTabs[i], doc,
+                    closedTabs[i].title, fragment);
+      }
+
+    createRestoreAllEntry(doc, fragment, aPrefixRestoreAll, false,
+                          aRestoreAllLabel, closedTabs.length, aTagName)
+    }
+    return fragment;
+  },
+
+  /**
+  * Builds up a document fragment of UI items for the recently closed windows.
+  * @param   aWindow
+  *          A window that can be used to create the elements and document fragment.
+  * @param   aTagName
+  *          The tag name that will be used when creating the UI items.
+  * @param   aPrefixRestoreAll (defaults to false)
+  *          Whether the 'restore all windows' item is suffixed or prefixed to the list.
+  *          If suffixed (the default) a separator will be inserted before it.
+  * @param   aRestoreAllLabel (defaults to "menuRestoreAllWindows.label")
+  *          Which localizable string to use for the 'restore all windows' item.
+  * @returns A document fragment with UI items for each recently closed window.
+  */
+  getWindowsFragment: function(aWindow, aTagName, aPrefixRestoreAll=false,
+                            aRestoreAllLabel="menuRestoreAllWindows.label") {
+    let closedWindowData = SessionStore.getClosedWindowData(false);
+    let doc = aWindow.document;
+    let fragment = doc.createDocumentFragment();
+    if (closedWindowData.length != 0) {
+      let menuLabelString = navigatorBundle.GetStringFromName("menuUndoCloseWindowLabel");
+      let menuLabelStringSingleTab =
+        navigatorBundle.GetStringFromName("menuUndoCloseWindowSingleTabLabel");
+
+      for (let i = 0; i < closedWindowData.length; i++) {
+        let undoItem = closedWindowData[i];
+        let otherTabsCount = undoItem.tabs.length - 1;
+        let label = (otherTabsCount == 0) ? menuLabelStringSingleTab
+                                          : PluralForm.get(otherTabsCount, menuLabelString);
+        let menuLabel = label.replace("#1", undoItem.title)
+                             .replace("#2", otherTabsCount);
+        let selectedTab = undoItem.tabs[undoItem.selected - 1];
+
+        createEntry(aTagName, true, i, selectedTab, doc, menuLabel,
+                    fragment);
+      }
+
+      createRestoreAllEntry(doc, fragment, aPrefixRestoreAll, true,
+                            aRestoreAllLabel, closedWindowData.length,
+                            aTagName);
+    }
+    return fragment;
+  },
+
+
+  /**
+    * Re-open a closed tab and put it to the end of the tab strip.
+    * Used for a middle click.
+    * @param aEvent
+    *        The event when the user clicks the menu item
+    */
+  _undoCloseMiddleClick: function(aEvent) {
+    if (aEvent.button != 1)
+      return;
+
+    aEvent.view.undoCloseTab(aEvent.originalTarget.getAttribute("value"));
+    aEvent.view.gBrowser.moveTabToEnd();
+  },
+};
+
+function setImage(aItem, aElement) {
+  let iconURL = aItem.image;
+  // don't initiate a connection just to fetch a favicon (see bug 467828)
+  if (/^https?:/.test(iconURL))
+    iconURL = "moz-anno:favicon:" + iconURL;
+
+  aElement.setAttribute("image", iconURL);
+}
+
+/**
+ * Create a UI entry for a recently closed tab or window.
+ * @param aTagName
+ *        the tag name that will be used when creating the UI entry
+ * @param aIsWindowsFragment
+ *        whether or not this entry will represent a closed window
+ * @param aIndex
+ *        the index of the closed tab
+ * @param aClosedTab
+ *        the closed tab
+ * @param aDocument
+ *        a document that can be used to create the entry
+ * @param aMenuLabel
+ *        the label the created entry will have
+ * @param aFragment
+ *        the fragment the created entry will be in
+ */
+function createEntry(aTagName, aIsWindowsFragment, aIndex, aClosedTab,
+                     aDocument, aMenuLabel, aFragment) {
+  let element = aDocument.createElementNS(kNSXUL, aTagName);
+
+  element.setAttribute("label", aMenuLabel);
+  if (aClosedTab.image) {
+    setImage(aClosedTab, element);
+  }
+  if (!aIsWindowsFragment) {
+    element.setAttribute("value", aIndex);
+  }
+
+  if (aTagName == "menuitem") {
+    element.setAttribute("class", "menuitem-iconic bookmark-item menuitem-with-favicon");
+  }
+
+  element.setAttribute("oncommand", "undoClose" + (aIsWindowsFragment ? "Window" : "Tab") +
+                       "(" + aIndex + ");");
+
+  // Set the targetURI attribute so it will be shown in tooltip.
+  // SessionStore uses one-based indexes, so we need to normalize them.
+  let tabData;
+  tabData = aIsWindowsFragment ? aClosedTab
+                     : aClosedTab.state;
+  let activeIndex = (tabData.index || tabData.entries.length) - 1;
+  if (activeIndex >= 0 && tabData.entries[activeIndex]) {
+    element.setAttribute("targetURI", tabData.entries[activeIndex].url);
+  }
+
+  if (!aIsWindowsFragment) {
+    element.addEventListener("click", RecentlyClosedTabsAndWindowsMenuUtils._undoCloseMiddleClick, false);
+  }
+  if (aIndex == 0) {
+    element.setAttribute("key", "key_undoClose" + (aIsWindowsFragment? "Window" : "Tab"));
+  }
+
+  aFragment.appendChild(element);
+}
+
+/**
+ * Create an entry to restore all closed windows or tabs.
+ * @param aDocument
+ *        a document that can be used to create the entry
+ * @param aFragment
+ *        the fragment the created entry will be in
+ * @param aPrefixRestoreAll
+ *        whether the 'restore all windows' item is suffixed or prefixed to the list
+ *        If suffixed a separator will be inserted before it.
+ * @param aIsWindowsFragment
+ *        whether or not this entry will represent a closed window
+ * @param aRestoreAllLabel
+ *        which localizable string to use for the entry
+ * @param aEntryCount
+ *        the number of elements to be restored by this entry
+ * @param aTagName
+ *        the tag name that will be used when creating the UI entry
+ */
+function createRestoreAllEntry(aDocument, aFragment, aPrefixRestoreAll,
+                                aIsWindowsFragment, aRestoreAllLabel,
+                                aEntryCount, aTagName) {
+  let restoreAllElements = aDocument.createElementNS(kNSXUL, aTagName);
+  restoreAllElements.classList.add("restoreallitem");
+  restoreAllElements.setAttribute("label", navigatorBundle.GetStringFromName(aRestoreAllLabel));
+  restoreAllElements.setAttribute("oncommand",
+                                  "for (var i = 0; i < " + aEntryCount + "; i++) undoClose" +
+                                    (aIsWindowsFragment? "Window" : "Tab") + "();");
+  if (aPrefixRestoreAll) {
+    aFragment.insertBefore(restoreAllElements, aFragment.firstChild);
+  } else {
+    aFragment.appendChild(aDocument.createElementNS(kNSXUL, "menuseparator"));
+    aFragment.appendChild(restoreAllElements);
+  }
+}
\ No newline at end of file
diff --git a/application/basilisk/components/sessionstore/RunState.jsm b/application/basilisk/components/sessionstore/RunState.jsm
new file mode 100644
index 0000000000..3cdf47718a
--- /dev/null
+++ b/application/basilisk/components/sessionstore/RunState.jsm
@@ -0,0 +1,96 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this file,
+ * You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+"use strict";
+
+this.EXPORTED_SYMBOLS = ["RunState"];
+
+const STATE_STOPPED = 0;
+const STATE_RUNNING = 1;
+const STATE_QUITTING = 2;
+const STATE_CLOSING = 3;
+const STATE_CLOSED = 4;
+
+// We're initially stopped.
+var state = STATE_STOPPED;
+
+/**
+ * This module keeps track of SessionStore's current run state. We will
+ * always start out at STATE_STOPPED. After the session was read from disk and
+ * the initial browser window has loaded we switch to STATE_RUNNING. On the
+ * first notice that a browser shutdown was granted we switch to STATE_QUITTING.
+ */
+this.RunState = Object.freeze({
+  // If we're stopped then SessionStore hasn't been initialized yet. As soon
+  // as the session is read from disk and the initial browser window has loaded
+  // the run state will change to STATE_RUNNING.
+  get isStopped() {
+    return state == STATE_STOPPED;
+  },
+
+  // STATE_RUNNING is our default mode of operation that we'll spend most of
+  // the time in. After the session was read from disk and the first browser
+  // window has loaded we remain running until the browser quits.
+  get isRunning() {
+    return state == STATE_RUNNING;
+  },
+
+  // We will enter STATE_QUITTING as soon as we receive notice that a browser
+  // shutdown was granted. SessionStore will use this information to prevent
+  // us from collecting partial information while the browser is shutting down
+  // as well as to allow a last single write to disk and block all writes after
+  // that.
+  get isQuitting() {
+    return state >= STATE_QUITTING;
+  },
+
+  // We will enter STATE_CLOSING as soon as SessionStore is uninitialized.
+  // The SessionFile module will know that a last write will happen in this
+  // state and it can do some necessary cleanup.
+  get isClosing() {
+    return state == STATE_CLOSING;
+  },
+
+  // We will enter STATE_CLOSED as soon as SessionFile has written to disk for
+  // the last time before shutdown and will not accept any further writes.
+  get isClosed() {
+    return state == STATE_CLOSED;
+  },
+
+  // Switch the run state to STATE_RUNNING. This must be called after the
+  // session was read from, the initial browser window has loaded and we're
+  // now ready to restore session data.
+  setRunning() {
+    if (this.isStopped) {
+      state = STATE_RUNNING;
+    }
+  },
+
+  // Switch the run state to STATE_CLOSING. This must be called *before* the
+  // last SessionFile.write() call so that SessionFile knows we're closing and
+  // can do some last cleanups and write a proper sessionstore.js file.
+  setClosing() {
+    if (this.isQuitting) {
+      state = STATE_CLOSING;
+    }
+  },
+
+  // Switch the run state to STATE_CLOSED. This must be called by SessionFile
+  // after the last write to disk was accepted and no further writes will be
+  // allowed. Any writes after this stage will cause exceptions.
+  setClosed() {
+    if (this.isClosing) {
+      state = STATE_CLOSED;
+    }
+  },
+
+  // Switch the run state to STATE_QUITTING. This should be called once we're
+  // certain that the browser is going away and before we start collecting the
+  // final window states to save in the session file.
+  setQuitting() {
+    if (this.isRunning) {
+      state = STATE_QUITTING;
+    }
+  },
+});
diff --git a/application/basilisk/components/sessionstore/SessionCookies.jsm b/application/basilisk/components/sessionstore/SessionCookies.jsm
new file mode 100644
index 0000000000..b99ab927be
--- /dev/null
+++ b/application/basilisk/components/sessionstore/SessionCookies.jsm
@@ -0,0 +1,476 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this file,
+ * You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+"use strict";
+
+this.EXPORTED_SYMBOLS = ["SessionCookies"];
+
+const Cu = Components.utils;
+const Ci = Components.interfaces;
+
+Cu.import("resource://gre/modules/Services.jsm", this);
+Cu.import("resource://gre/modules/XPCOMUtils.jsm", this);
+
+XPCOMUtils.defineLazyModuleGetter(this, "Utils",
+  "resource://gre/modules/sessionstore/Utils.jsm");
+XPCOMUtils.defineLazyModuleGetter(this, "PrivacyLevel",
+  "resource:///modules/sessionstore/PrivacyLevel.jsm");
+
+// MAX_EXPIRY should be 2^63-1, but JavaScript can't handle that precision.
+const MAX_EXPIRY = Math.pow(2, 62);
+
+/**
+ * The external API implemented by the SessionCookies module.
+ */
+this.SessionCookies = Object.freeze({
+  update: function (windows) {
+    SessionCookiesInternal.update(windows);
+  },
+
+  getHostsForWindow: function (window, checkPrivacy = false) {
+    return SessionCookiesInternal.getHostsForWindow(window, checkPrivacy);
+  },
+
+  restore(cookies) {
+    SessionCookiesInternal.restore(cookies);
+  }
+});
+
+/**
+ * The internal API.
+ */
+var SessionCookiesInternal = {
+  /**
+   * Stores whether we're initialized, yet.
+   */
+  _initialized: false,
+
+  /**
+   * Retrieve the list of all hosts contained in the given windows' session
+   * history entries (per window) and collect the associated cookies for those
+   * hosts, if any. The given state object is being modified.
+   *
+   * @param windows
+   *        Array of window state objects.
+   *        [{ tabs: [...], cookies: [...] }, ...]
+   */
+  update: function (windows) {
+    this._ensureInitialized();
+
+    for (let window of windows) {
+      let cookies = [];
+
+      // Collect all hosts for the current window.
+      let hosts = this.getHostsForWindow(window, true);
+
+      for (let host of Object.keys(hosts)) {
+        let isPinned = hosts[host];
+
+        for (let cookie of CookieStore.getCookiesForHost(host)) {
+          // _getCookiesForHost() will only return hosts with the right privacy
+          // rules, so there is no need to do anything special with this call
+          // to PrivacyLevel.canSave().
+          if (PrivacyLevel.canSave({isHttps: cookie.secure, isPinned: isPinned})) {
+            cookies.push(cookie);
+          }
+        }
+      }
+
+      // Don't include/keep empty cookie sections.
+      if (cookies.length) {
+        window.cookies = cookies;
+      } else if ("cookies" in window) {
+        delete window.cookies;
+      }
+    }
+  },
+
+  /**
+   * Returns a map of all hosts for a given window that we might want to
+   * collect cookies for.
+   *
+   * @param window
+   *        A window state object containing tabs with history entries.
+   * @param checkPrivacy (bool)
+   *        Whether to check the privacy level for each host.
+   * @return {object} A map of hosts for a given window state object. The keys
+   *                  will be hosts, the values are boolean and determine
+   *                  whether we will use the deferred privacy level when
+   *                  checking how much data to save on quitting.
+   */
+  getHostsForWindow: function (window, checkPrivacy = false) {
+    let hosts = {};
+
+    for (let tab of window.tabs) {
+      for (let entry of tab.entries) {
+        this._extractHostsFromEntry(entry, hosts, checkPrivacy, tab.pinned);
+      }
+    }
+
+    return hosts;
+  },
+
+  /**
+   * Restores a given list of session cookies.
+   */
+  restore(cookies) {
+
+    for (let cookie of cookies) {
+      let expiry = "expiry" in cookie ? cookie.expiry : MAX_EXPIRY;
+      let cookieObj = {
+        host: cookie.host,
+        path: cookie.path || "",
+        name: cookie.name || ""
+      };
+      if (!Services.cookies.cookieExists(cookieObj, cookie.originAttributes || {})) {
+        Services.cookies.add(cookie.host, cookie.path || "", cookie.name || "",
+                             cookie.value, !!cookie.secure, !!cookie.httponly,
+                             /* isSession = */ true, expiry, cookie.originAttributes || {});
+      }
+    }
+  },
+
+  /**
+   * Handles observers notifications that are sent whenever cookies are added,
+   * changed, or removed. Ensures that the storage is updated accordingly.
+   */
+  observe: function (subject, topic, data) {
+    switch (data) {
+      case "added":
+      case "changed":
+        this._updateCookie(subject);
+        break;
+      case "deleted":
+        this._removeCookie(subject);
+        break;
+      case "cleared":
+        CookieStore.clear();
+        break;
+      case "batch-deleted":
+        this._removeCookies(subject);
+        break;
+      case "reload":
+        CookieStore.clear();
+        this._reloadCookies();
+        break;
+      default:
+        throw new Error("Unhandled cookie-changed notification.");
+    }
+  },
+
+  /**
+   * If called for the first time in a session, iterates all cookies in the
+   * cookies service and puts them into the store if they're session cookies.
+   */
+  _ensureInitialized: function () {
+    if (!this._initialized) {
+      this._reloadCookies();
+      this._initialized = true;
+      Services.obs.addObserver(this, "cookie-changed", false);
+    }
+  },
+
+  /**
+   * Fill a given map with hosts found in the given entry's session history and
+   * any child entries.
+   *
+   * @param entry
+   *        the history entry, serialized
+   * @param hosts
+   *        the hash that will be used to store hosts eg, { hostname: true }
+   * @param checkPrivacy
+   *        should we check the privacy level for https
+   * @param isPinned
+   *        is the entry we're evaluating for a pinned tab; used only if
+   *        checkPrivacy
+   */
+  _extractHostsFromEntry: function (entry, hosts, checkPrivacy, isPinned) {
+    let host = entry._host;
+    let scheme = entry._scheme;
+
+    // If host & scheme aren't defined, then we are likely here in the startup
+    // process via _splitCookiesFromWindow. In that case, we'll turn entry.url
+    // into an nsIURI and get host/scheme from that. This will throw for about:
+    // urls in which case we don't need to do anything.
+    if (!host && !scheme) {
+      try {
+        let uri = Utils.makeURI(entry.url);
+        host = uri.host;
+        scheme = uri.scheme;
+        this._extractHostsFromHostScheme(host, scheme, hosts, checkPrivacy, isPinned);
+      }
+      catch (ex) { }
+    }
+
+    if (entry.children) {
+      for (let child of entry.children) {
+        this._extractHostsFromEntry(child, hosts, checkPrivacy, isPinned);
+      }
+    }
+  },
+
+  /**
+   * Add a given host to a given map of hosts if the privacy level allows
+   * saving cookie data for it.
+   *
+   * @param host
+   *        the host of a uri (usually via nsIURI.host)
+   * @param scheme
+   *        the scheme of a uri (usually via nsIURI.scheme)
+   * @param hosts
+   *        the hash that will be used to store hosts eg, { hostname: true }
+   * @param checkPrivacy
+   *        should we check the privacy level for https
+   * @param isPinned
+   *        is the entry we're evaluating for a pinned tab; used only if
+   *        checkPrivacy
+   */
+  _extractHostsFromHostScheme:
+    function (host, scheme, hosts, checkPrivacy, isPinned) {
+    // host and scheme may not be set (for about: urls for example), in which
+    // case testing scheme will be sufficient.
+    if (/https?/.test(scheme) && !hosts[host] &&
+        (!checkPrivacy ||
+         PrivacyLevel.canSave({isHttps: scheme == "https", isPinned: isPinned}))) {
+      // By setting this to true or false, we can determine when looking at
+      // the host in update() if we should check for privacy.
+      hosts[host] = isPinned;
+    } else if (scheme == "file") {
+      hosts[host] = true;
+    }
+  },
+
+  /**
+   * Updates or adds a given cookie to the store.
+   */
+  _updateCookie: function (cookie) {
+    cookie.QueryInterface(Ci.nsICookie2);
+
+    if (cookie.isSession) {
+      CookieStore.set(cookie);
+    } else {
+      CookieStore.delete(cookie);
+    }
+  },
+
+  /**
+   * Removes a given cookie from the store.
+   */
+  _removeCookie: function (cookie) {
+    cookie.QueryInterface(Ci.nsICookie2);
+
+    if (cookie.isSession) {
+      CookieStore.delete(cookie);
+    }
+  },
+
+  /**
+   * Removes a given list of cookies from the store.
+   */
+  _removeCookies: function (cookies) {
+    for (let i = 0; i < cookies.length; i++) {
+      this._removeCookie(cookies.queryElementAt(i, Ci.nsICookie2));
+    }
+  },
+
+  /**
+   * Iterates all cookies in the cookies service and puts them into the store
+   * if they're session cookies.
+   */
+  _reloadCookies: function () {
+    let iter = Services.cookies.enumerator;
+    while (iter.hasMoreElements()) {
+      this._updateCookie(iter.getNext());
+    }
+  }
+};
+
+/**
+ * Generates all possible subdomains for a given host and prepends a leading
+ * dot to all variants.
+ *
+ * See http://tools.ietf.org/html/rfc6265#section-5.1.3
+ *     http://en.wikipedia.org/wiki/HTTP_cookie#Domain_and_Path
+ *
+ * All cookies belonging to a web page will be internally represented by a
+ * nsICookie object. nsICookie.host will be the request host if no domain
+ * parameter was given when setting the cookie. If a specific domain was given
+ * then nsICookie.host will contain that specific domain and prepend a leading
+ * dot to it.
+ *
+ * We thus generate all possible subdomains for a given domain and prepend a
+ * leading dot to them as that is the value that was used as the map key when
+ * the cookie was set.
+ */
+function* getPossibleSubdomainVariants(host) {
+  // Try given domain with a leading dot (.www.example.com).
+  yield "." + host;
+
+  // Stop if there are only two parts left (e.g. example.com was given).
+  let parts = host.split(".");
+  if (parts.length < 3) {
+    return;
+  }
+
+  // Remove the first subdomain (www.example.com -> example.com).
+  let rest = parts.slice(1).join(".");
+
+  // Try possible parent subdomains.
+  yield* getPossibleSubdomainVariants(rest);
+}
+
+/**
+ * The internal cookie storage that keeps track of every active session cookie.
+ * These are stored using maps per host, path, and cookie name.
+ */
+var CookieStore = {
+  /**
+   * The internal structure holding all known cookies.
+   *
+   * Host =>
+   *  Path =>
+   *    Name => {path: "/", name: "sessionid", secure: true}
+   *
+   * Maps are used for storage but the data structure is equivalent to this:
+   *
+   * this._hosts = {
+   *   "www.mozilla.org": {
+   *     "/": {
+   *       "username": {name: "username", value: "my_name_is", etc...},
+   *       "sessionid": {name: "sessionid", value: "1fdb3a", etc...}
+   *     }
+   *   },
+   *   "tbpl.mozilla.org": {
+   *     "/path": {
+   *       "cookiename": {name: "cookiename", value: "value", etc...}
+   *     }
+   *   },
+   *   ".example.com": {
+   *     "/path": {
+   *       "cookiename": {name: "cookiename", value: "value", etc...}
+   *     }
+   *   }
+   * };
+   */
+  _hosts: new Map(),
+
+  /**
+   * Returns the list of stored session cookies for a given host.
+   *
+   * @param host
+   *        A string containing the host name we want to get cookies for.
+   */
+  getCookiesForHost: function (host) {
+    let cookies = [];
+
+    let appendCookiesForHost = host => {
+      if (!this._hosts.has(host)) {
+        return;
+      }
+
+      for (let pathToNamesMap of this._hosts.get(host).values()) {
+        for (let nameToCookiesMap of pathToNamesMap.values()) {
+          cookies.push(...nameToCookiesMap.values());
+        }
+      }
+    }
+
+    // Try to find cookies for the given host, e.g. <www.example.com>.
+    // The full hostname will be in the map if the Set-Cookie header did not
+    // have a domain= attribute, i.e. the cookie will only be stored for the
+    // request domain. Also, try to find cookies for subdomains, e.g.
+    // <.example.com>. We will find those variants with a leading dot in the
+    // map if the Set-Cookie header had a domain= attribute, i.e. the cookie
+    // will be stored for a parent domain and we send it for any subdomain.
+    for (let variant of [host, ...getPossibleSubdomainVariants(host)]) {
+      appendCookiesForHost(variant);
+    }
+
+    return cookies;
+  },
+
+  /**
+   * Stores a given cookie.
+   *
+   * @param cookie
+   *        The nsICookie2 object to add to the storage.
+   */
+  set: function (cookie) {
+    let jscookie = {host: cookie.host, value: cookie.value};
+
+    // Only add properties with non-default values to save a few bytes.
+    if (cookie.path) {
+      jscookie.path = cookie.path;
+    }
+
+    if (cookie.name) {
+      jscookie.name = cookie.name;
+    }
+
+    if (cookie.isSecure) {
+      jscookie.secure = true;
+    }
+
+    if (cookie.isHttpOnly) {
+      jscookie.httponly = true;
+    }
+
+    if (cookie.expiry < MAX_EXPIRY) {
+      jscookie.expiry = cookie.expiry;
+    }
+
+    if (cookie.originAttributes) {
+      jscookie.originAttributes = cookie.originAttributes;
+    }
+
+    this._ensureMap(cookie).set(cookie.name, jscookie);
+  },
+
+  /**
+   * Removes a given cookie.
+   *
+   * @param cookie
+   *        The nsICookie2 object to be removed from storage.
+   */
+  delete: function (cookie) {
+    this._ensureMap(cookie).delete(cookie.name);
+  },
+
+  /**
+   * Removes all cookies.
+   */
+  clear: function () {
+    this._hosts.clear();
+  },
+
+  /**
+   * Creates all maps necessary to store a given cookie.
+   *
+   * @param cookie
+   *        The nsICookie2 object to create maps for.
+   *
+   * @return The newly created Map instance mapping cookie names to
+   *         internal jscookies, in the given path of the given host.
+   */
+  _ensureMap: function (cookie) {
+    if (!this._hosts.has(cookie.host)) {
+      this._hosts.set(cookie.host, new Map());
+    }
+
+    let originAttributesMap = this._hosts.get(cookie.host);
+    // If cookie.originAttributes is null, originAttributes will be an empty string.
+    let originAttributes = ChromeUtils.originAttributesToSuffix(cookie.originAttributes);
+    if (!originAttributesMap.has(originAttributes)) {
+      originAttributesMap.set(originAttributes, new Map());
+    }
+
+    let pathToNamesMap = originAttributesMap.get(originAttributes);
+
+    if (!pathToNamesMap.has(cookie.path)) {
+      pathToNamesMap.set(cookie.path, new Map());
+    }
+
+    return pathToNamesMap.get(cookie.path);
+  }
+};
diff --git a/application/basilisk/components/sessionstore/SessionFile.jsm b/application/basilisk/components/sessionstore/SessionFile.jsm
new file mode 100644
index 0000000000..80c4e77904
--- /dev/null
+++ b/application/basilisk/components/sessionstore/SessionFile.jsm
@@ -0,0 +1,399 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this file,
+ * You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+"use strict";
+
+this.EXPORTED_SYMBOLS = ["SessionFile"];
+
+/**
+ * Implementation of all the disk I/O required by the session store.
+ * This is a private API, meant to be used only by the session store.
+ * It will change. Do not use it for any other purpose.
+ *
+ * Note that this module implicitly depends on one of two things:
+ * 1. either the asynchronous file I/O system enqueues its requests
+ *   and never attempts to simultaneously execute two I/O requests on
+ *   the files used by this module from two distinct threads; or
+ * 2. the clients of this API are well-behaved and do not place
+ *   concurrent requests to the files used by this module.
+ *
+ * Otherwise, we could encounter bugs, especially under Windows,
+ *   e.g. if a request attempts to write sessionstore.js while
+ *   another attempts to copy that file.
+ *
+ * This implementation uses OS.File, which guarantees property 1.
+ */
+
+const Cu = Components.utils;
+const Cc = Components.classes;
+const Ci = Components.interfaces;
+const Cr = Components.results;
+
+Cu.import("resource://gre/modules/Services.jsm");
+Cu.import("resource://gre/modules/XPCOMUtils.jsm");
+Cu.import("resource://gre/modules/osfile.jsm");
+Cu.import("resource://gre/modules/Promise.jsm");
+Cu.import("resource://gre/modules/AsyncShutdown.jsm");
+Cu.import("resource://gre/modules/Preferences.jsm");
+
+XPCOMUtils.defineLazyModuleGetter(this, "console",
+  "resource://gre/modules/Console.jsm");
+XPCOMUtils.defineLazyModuleGetter(this, "PromiseUtils",
+  "resource://gre/modules/PromiseUtils.jsm");
+XPCOMUtils.defineLazyModuleGetter(this, "RunState",
+  "resource:///modules/sessionstore/RunState.jsm");
+XPCOMUtils.defineLazyModuleGetter(this, "TelemetryStopwatch",
+  "resource://gre/modules/TelemetryStopwatch.jsm");
+XPCOMUtils.defineLazyModuleGetter(this, "Task",
+  "resource://gre/modules/Task.jsm");
+XPCOMUtils.defineLazyServiceGetter(this, "Telemetry",
+  "@mozilla.org/base/telemetry;1", "nsITelemetry");
+XPCOMUtils.defineLazyServiceGetter(this, "sessionStartup",
+  "@mozilla.org/browser/sessionstartup;1", "nsISessionStartup");
+XPCOMUtils.defineLazyModuleGetter(this, "SessionWorker",
+  "resource:///modules/sessionstore/SessionWorker.jsm");
+XPCOMUtils.defineLazyModuleGetter(this, "SessionStore",
+  "resource:///modules/sessionstore/SessionStore.jsm");
+
+const PREF_UPGRADE_BACKUP = "browser.sessionstore.upgradeBackup.latestBuildID";
+const PREF_MAX_UPGRADE_BACKUPS = "browser.sessionstore.upgradeBackup.maxUpgradeBackups";
+
+const PREF_MAX_SERIALIZE_BACK = "browser.sessionstore.max_serialize_back";
+const PREF_MAX_SERIALIZE_FWD = "browser.sessionstore.max_serialize_forward";
+
+this.SessionFile = {
+  /**
+   * Read the contents of the session file, asynchronously.
+   */
+  read: function () {
+    return SessionFileInternal.read();
+  },
+  /**
+   * Write the contents of the session file, asynchronously.
+   */
+  write: function (aData) {
+    return SessionFileInternal.write(aData);
+  },
+  /**
+   * Wipe the contents of the session file, asynchronously.
+   */
+  wipe: function () {
+    return SessionFileInternal.wipe();
+  },
+
+  /**
+   * Return the paths to the files used to store, backup, etc.
+   * the state of the file.
+   */
+  get Paths() {
+    return SessionFileInternal.Paths;
+  }
+};
+
+Object.freeze(SessionFile);
+
+var Path = OS.Path;
+var profileDir = OS.Constants.Path.profileDir;
+
+var SessionFileInternal = {
+  Paths: Object.freeze({
+    // The path to the latest version of sessionstore written during a clean
+    // shutdown. After startup, it is renamed `cleanBackup`.
+    clean: Path.join(profileDir, "sessionstore.js"),
+
+    // The path at which we store the previous version of `clean`. Updated
+    // whenever we successfully load from `clean`.
+    cleanBackup: Path.join(profileDir, "sessionstore-backups", "previous.js"),
+
+    // The directory containing all sessionstore backups.
+    backups: Path.join(profileDir, "sessionstore-backups"),
+
+    // The path to the latest version of the sessionstore written
+    // during runtime. Generally, this file contains more
+    // privacy-sensitive information than |clean|, and this file is
+    // therefore removed during clean shutdown. This file is designed to protect
+    // against crashes / sudden shutdown.
+    recovery: Path.join(profileDir, "sessionstore-backups", "recovery.js"),
+
+    // The path to the previous version of the sessionstore written
+    // during runtime (e.g. 15 seconds before recovery). In case of a
+    // clean shutdown, this file is removed.  Generally, this file
+    // contains more privacy-sensitive information than |clean|, and
+    // this file is therefore removed during clean shutdown.  This
+    // file is designed to protect against crashes that are nasty
+    // enough to corrupt |recovery|.
+    recoveryBackup: Path.join(profileDir, "sessionstore-backups", "recovery.bak"),
+
+    // The path to a backup created during an upgrade of Firefox.
+    // Having this backup protects the user essentially from bugs in
+    // Firefox or add-ons, especially for users of Nightly. This file
+    // does not contain any information more sensitive than |clean|.
+    upgradeBackupPrefix: Path.join(profileDir, "sessionstore-backups", "upgrade.js-"),
+
+    // The path to the backup of the version of the session store used
+    // during the latest upgrade of Firefox. During load/recovery,
+    // this file should be used if both |path|, |backupPath| and
+    // |latestStartPath| are absent/incorrect.  May be "" if no
+    // upgrade backup has ever been performed. This file does not
+    // contain any information more sensitive than |clean|.
+    get upgradeBackup() {
+      let latestBackupID = SessionFileInternal.latestUpgradeBackupID;
+      if (!latestBackupID) {
+        return "";
+      }
+      return this.upgradeBackupPrefix + latestBackupID;
+    },
+
+    // The path to a backup created during an upgrade of Firefox.
+    // Having this backup protects the user essentially from bugs in
+    // Firefox, especially for users of Nightly.
+    get nextUpgradeBackup() {
+      return this.upgradeBackupPrefix + Services.appinfo.platformBuildID;
+    },
+
+    /**
+     * The order in which to search for a valid sessionstore file.
+     */
+    get loadOrder() {
+      // If `clean` exists and has been written without corruption during
+      // the latest shutdown, we need to use it.
+      //
+      // Otherwise, `recovery` and `recoveryBackup` represent the most
+      // recent state of the session store.
+      //
+      // Finally, if nothing works, fall back to the last known state
+      // that can be loaded (`cleanBackup`) or, if available, to the
+      // backup performed during the latest upgrade.
+      let order = ["clean",
+                   "recovery",
+                   "recoveryBackup",
+                   "cleanBackup"];
+      if (SessionFileInternal.latestUpgradeBackupID) {
+        // We have an upgradeBackup
+        order.push("upgradeBackup");
+      }
+      return order;
+    },
+  }),
+
+  // Number of attempted calls to `write`.
+  // Note that we may have _attempts > _successes + _failures,
+  // if attempts never complete.
+  // Used for error reporting.
+  _attempts: 0,
+
+  // Number of successful calls to `write`.
+  // Used for error reporting.
+  _successes: 0,
+
+  // Number of failed calls to `write`.
+  // Used for error reporting.
+  _failures: 0,
+
+  // Resolved once initialization is complete.
+  // The promise never rejects.
+  _deferredInitialized: PromiseUtils.defer(),
+
+  // `true` once we have started initialization, i.e. once something
+  // has been scheduled that will eventually resolve `_deferredInitialized`.
+  _initializationStarted: false,
+
+  // The ID of the latest version of Gecko for which we have an upgrade backup
+  // or |undefined| if no upgrade backup was ever written.
+  get latestUpgradeBackupID() {
+    try {
+      return Services.prefs.getCharPref(PREF_UPGRADE_BACKUP);
+    } catch (ex) {
+      return undefined;
+    }
+  },
+
+  // Find the correct session file, read it and setup the worker.
+  read: Task.async(function* () {
+    this._initializationStarted = true;
+
+    let result;
+    let noFilesFound = true;
+    // Attempt to load by order of priority from the various backups
+    for (let key of this.Paths.loadOrder) {
+      let corrupted = false;
+      let exists = true;
+      try {
+        let path = this.Paths[key];
+        let startMs = Date.now();
+
+        let source = yield OS.File.read(path, { encoding: "utf-8" });
+        let parsed = JSON.parse(source);
+
+        if (!SessionStore.isFormatVersionCompatible(parsed.version || ["sessionrestore", 0] /*fallback for old versions*/)) {
+          // Skip sessionstore files that we don't understand.
+          Cu.reportError("Cannot extract data from Session Restore file " + path + ". Wrong format/version: " + JSON.stringify(parsed.version) + ".");
+          continue;
+        }
+        result = {
+          origin: key,
+          source: source,
+          parsed: parsed
+        };
+        Telemetry.getHistogramById("FX_SESSION_RESTORE_CORRUPT_FILE").
+          add(false);
+        Telemetry.getHistogramById("FX_SESSION_RESTORE_READ_FILE_MS").
+          add(Date.now() - startMs);
+        break;
+      } catch (ex if ex instanceof OS.File.Error && ex.becauseNoSuchFile) {
+        exists = false;
+      } catch (ex if ex instanceof OS.File.Error) {
+        // The file might be inaccessible due to wrong permissions
+        // or similar failures. We'll just count it as "corrupted".
+        console.error("Could not read session file ", ex, ex.stack);
+        corrupted = true;
+      } catch (ex if ex instanceof SyntaxError) {
+        console.error("Corrupt session file (invalid JSON found) ", ex, ex.stack);
+        // File is corrupted, try next file
+        corrupted = true;
+      } finally {
+        if (exists) {
+          noFilesFound = false;
+          Telemetry.getHistogramById("FX_SESSION_RESTORE_CORRUPT_FILE").
+            add(corrupted);
+        }
+      }
+    }
+
+    // All files are corrupted if files found but none could deliver a result.
+    let allCorrupt = !noFilesFound && !result;
+    Telemetry.getHistogramById("FX_SESSION_RESTORE_ALL_FILES_CORRUPT").
+      add(allCorrupt);
+
+    if (!result) {
+      // If everything fails, start with an empty session.
+      result = {
+        origin: "empty",
+        source: "",
+        parsed: null
+      };
+    }
+
+    result.noFilesFound = noFilesFound;
+
+    // Initialize the worker (in the background) to let it handle backups and also
+    // as a workaround for bug 964531.
+    let promiseInitialized = SessionWorker.post("init", [result.origin, this.Paths, {
+      maxUpgradeBackups: Preferences.get(PREF_MAX_UPGRADE_BACKUPS, 3),
+      maxSerializeBack: Preferences.get(PREF_MAX_SERIALIZE_BACK, 10),
+      maxSerializeForward: Preferences.get(PREF_MAX_SERIALIZE_FWD, -1)
+    }]);
+
+    promiseInitialized.catch(err => {
+      // Ensure that we report errors but that they do not stop us.
+      Promise.reject(err);
+    }).then(() => this._deferredInitialized.resolve());
+
+    return result;
+  }),
+
+  // Post a message to the worker, making sure that it has been initialized
+  // first.
+  _postToWorker: Task.async(function*(...args) {
+    if (!this._initializationStarted) {
+      // Initializing the worker is somewhat complex, as proper handling of
+      // backups requires us to first read and check the session. Consequently,
+      // the only way to initialize the worker is to first call `this.read()`.
+
+      // The call to `this.read()` causes background initialization of the worker.
+      // Initialization will be complete once `this._deferredInitialized.promise`
+      // resolves.
+      this.read();
+    }
+    yield this._deferredInitialized.promise;
+    return SessionWorker.post(...args)
+  }),
+
+  write: function (aData) {
+    if (RunState.isClosed) {
+      return Promise.reject(new Error("SessionFile is closed"));
+    }
+
+    let isFinalWrite = false;
+    if (RunState.isClosing) {
+      // If shutdown has started, we will want to stop receiving
+      // write instructions.
+      isFinalWrite = true;
+      RunState.setClosed();
+    }
+
+    let performShutdownCleanup = isFinalWrite &&
+      !sessionStartup.isAutomaticRestoreEnabled();
+
+    this._attempts++;
+    let options = {isFinalWrite, performShutdownCleanup};
+    let promise = this._postToWorker("write", [aData, options]);
+
+    // Wait until the write is done.
+    promise = promise.then(msg => {
+      // Record how long the write took.
+      this._recordTelemetry(msg.telemetry);
+      this._successes++;
+      if (msg.result.upgradeBackup) {
+        // We have just completed a backup-on-upgrade, store the information
+        // in preferences.
+        Services.prefs.setCharPref(PREF_UPGRADE_BACKUP,
+          Services.appinfo.platformBuildID);
+      }
+    }, err => {
+      // Catch and report any errors.
+      console.error("Could not write session state file ", err, err.stack);
+      this._failures++;
+      // By not doing anything special here we ensure that |promise| cannot
+      // be rejected anymore. The shutdown/cleanup code at the end of the
+      // function will thus always be executed.
+    });
+
+    // Ensure that we can write sessionstore.js cleanly before the profile
+    // becomes unaccessible.
+    AsyncShutdown.profileBeforeChange.addBlocker(
+      "SessionFile: Finish writing Session Restore data",
+      promise,
+      {
+        fetchState: () => ({
+          options,
+          attempts: this._attempts,
+          successes: this._successes,
+          failures: this._failures,
+        })
+      });
+
+    // This code will always be executed because |promise| can't fail anymore.
+    // We ensured that by having a reject handler that reports the failure but
+    // doesn't forward the rejection.
+    return promise.then(() => {
+      // Remove the blocker, no matter if writing failed or not.
+      AsyncShutdown.profileBeforeChange.removeBlocker(promise);
+
+      if (isFinalWrite) {
+        Services.obs.notifyObservers(null, "sessionstore-final-state-write-complete", "");
+      }
+    });
+  },
+
+  wipe: function () {
+    return this._postToWorker("wipe");
+  },
+
+  _recordTelemetry: function(telemetry) {
+    for (let id of Object.keys(telemetry)){
+      let value = telemetry[id];
+      let samples = [];
+      if (Array.isArray(value)) {
+        samples.push(...value);
+      } else {
+        samples.push(value);
+      }
+      let histogram = Telemetry.getHistogramById(id);
+      for (let sample of samples) {
+        histogram.add(sample);
+      }
+    }
+  }
+};
diff --git a/application/basilisk/components/sessionstore/SessionHistory.jsm b/application/basilisk/components/sessionstore/SessionHistory.jsm
new file mode 100644
index 0000000000..3d28d87db8
--- /dev/null
+++ b/application/basilisk/components/sessionstore/SessionHistory.jsm
@@ -0,0 +1,431 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+* License, v. 2.0. If a copy of the MPL was not distributed with this file,
+* You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+"use strict";
+
+this.EXPORTED_SYMBOLS = ["SessionHistory"];
+
+const Cu = Components.utils;
+const Cc = Components.classes;
+const Ci = Components.interfaces;
+
+Cu.import("resource://gre/modules/Services.jsm");
+Cu.import("resource://gre/modules/XPCOMUtils.jsm");
+
+XPCOMUtils.defineLazyModuleGetter(this, "Utils",
+  "resource://gre/modules/sessionstore/Utils.jsm");
+
+function debug(msg) {
+  Services.console.logStringMessage("SessionHistory: " + msg);
+}
+
+/**
+ * The external API exported by this module.
+ */
+this.SessionHistory = Object.freeze({
+  isEmpty: function (docShell) {
+    return SessionHistoryInternal.isEmpty(docShell);
+  },
+
+  collect: function (docShell) {
+    return SessionHistoryInternal.collect(docShell);
+  },
+
+  restore: function (docShell, tabData) {
+    SessionHistoryInternal.restore(docShell, tabData);
+  }
+});
+
+/**
+ * The internal API for the SessionHistory module.
+ */
+var SessionHistoryInternal = {
+  /**
+   * Returns whether the given docShell's session history is empty.
+   *
+   * @param docShell
+   *        The docShell that owns the session history.
+   */
+  isEmpty: function (docShell) {
+    let webNavigation = docShell.QueryInterface(Ci.nsIWebNavigation);
+    let history = webNavigation.sessionHistory;
+    if (!webNavigation.currentURI) {
+      return true;
+    }
+    let uri = webNavigation.currentURI.spec;
+    return uri == "about:blank" && history.count == 0;
+  },
+
+  /**
+   * Collects session history data for a given docShell.
+   *
+   * @param docShell
+   *        The docShell that owns the session history.
+   */
+  collect: function (docShell) {
+    let loadContext = docShell.QueryInterface(Ci.nsILoadContext);
+    let webNavigation = docShell.QueryInterface(Ci.nsIWebNavigation);
+    let history = webNavigation.sessionHistory.QueryInterface(Ci.nsISHistoryInternal);
+
+    let data = {entries: [], userContextId: loadContext.originAttributes.userContextId };
+
+    if (history && history.count > 0) {
+      // Loop over the transaction linked list directly so we can get the
+      // persist property for each transaction.
+      for (let txn = history.rootTransaction; txn; txn = txn.next) {
+        let entry = this.serializeEntry(txn.sHEntry);
+        entry.persist = txn.persist;
+        data.entries.push(entry);
+      }
+
+      // Ensure the index isn't out of bounds if an exception was thrown above.
+      data.index = Math.min(history.index + 1, data.entries.length);
+    }
+
+    // If either the session history isn't available yet or doesn't have any
+    // valid entries, make sure we at least include the current page.
+    if (data.entries.length == 0) {
+      let uri = webNavigation.currentURI.spec;
+      let body = webNavigation.document.body;
+      // We landed here because the history is inaccessible or there are no
+      // history entries. In that case we should at least record the docShell's
+      // current URL as a single history entry. If the URL is not about:blank
+      // or it's a blank tab that was modified (like a custom newtab page),
+      // record it. For about:blank we explicitly want an empty array without
+      // an 'index' property to denote that there are no history entries.
+      if (uri != "about:blank" || (body && body.hasChildNodes())) {
+        data.entries.push({
+          url: uri,
+          triggeringPrincipal_base64: Utils.SERIALIZED_SYSTEMPRINCIPAL
+        });
+        data.index = 1;
+      }
+    }
+
+    return data;
+  },
+
+  /**
+   * Get an object that is a serialized representation of a History entry.
+   *
+   * @param shEntry
+   *        nsISHEntry instance
+   * @return object
+   */
+  serializeEntry: function (shEntry) {
+    let entry = { url: shEntry.URI.spec };
+
+    // Save some bytes and don't include the title property
+    // if that's identical to the current entry's URL.
+    if (shEntry.title && shEntry.title != entry.url) {
+      entry.title = shEntry.title;
+    }
+    if (shEntry.isSubFrame) {
+      entry.subframe = true;
+    }
+
+    entry.charset = shEntry.URI.originCharset;
+
+    let cacheKey = shEntry.cacheKey;
+    if (cacheKey && cacheKey instanceof Ci.nsISupportsPRUint32 &&
+        cacheKey.data != 0) {
+      // XXXbz would be better to have cache keys implement
+      // nsISerializable or something.
+      entry.cacheKey = cacheKey.data;
+    }
+    entry.ID = shEntry.ID;
+    entry.docshellID = shEntry.docshellID;
+
+    // We will include the property only if it's truthy to save a couple of
+    // bytes when the resulting object is stringified and saved to disk.
+    if (shEntry.referrerURI) {
+      entry.referrer = shEntry.referrerURI.spec;
+      entry.referrerPolicy = shEntry.referrerPolicy;
+    }
+
+    if (shEntry.originalURI) {
+      entry.originalURI = shEntry.originalURI.spec;
+    }
+
+    if (shEntry.loadReplace) {
+      entry.loadReplace = shEntry.loadReplace;
+    }
+
+    if (shEntry.srcdocData)
+      entry.srcdocData = shEntry.srcdocData;
+
+    if (shEntry.isSrcdocEntry)
+      entry.isSrcdocEntry = shEntry.isSrcdocEntry;
+
+    if (shEntry.baseURI)
+      entry.baseURI = shEntry.baseURI.spec;
+
+    if (shEntry.contentType)
+      entry.contentType = shEntry.contentType;
+
+    if (shEntry.scrollRestorationIsManual) {
+      entry.scrollRestorationIsManual = true;
+    } else {
+      let x = {}, y = {};
+      shEntry.getScrollPosition(x, y);
+      if (x.value != 0 || y.value != 0)
+        entry.scroll = x.value + "," + y.value;
+    }
+
+    // Collect triggeringPrincipal data for the current history entry.
+    // Please note that before Bug 1297338 there was no concept of a
+    // principalToInherit. To remain backward/forward compatible we
+    // serialize the principalToInherit as triggeringPrincipal_b64.
+    // Once principalToInherit is well established (within FF55)
+    // we can update this code, remove triggeringPrincipal_b64 and
+    // just keep triggeringPrincipal_base64 as well as
+    // principalToInherit_base64;  see Bug 1301666.
+    if (shEntry.principalToInherit) {
+      try {
+        let principalToInherit = Utils.serializePrincipal(shEntry.principalToInherit);
+        if (principalToInherit) {
+          entry.triggeringPrincipal_b64 = principalToInherit;
+          entry.principalToInherit_base64 = principalToInherit;
+        }
+      } catch (e) {
+        debug(e);
+      }
+    }
+
+    if (shEntry.triggeringPrincipal) {
+      try {
+        let triggeringPrincipal = Utils.serializePrincipal(shEntry.triggeringPrincipal);
+        if (triggeringPrincipal) {
+          entry.triggeringPrincipal_base64 = triggeringPrincipal;
+        }
+      } catch (e) {
+        debug(e);
+      }
+    }
+
+    entry.docIdentifier = shEntry.BFCacheEntry.ID;
+
+    if (shEntry.stateData != null) {
+      entry.structuredCloneState = shEntry.stateData.getDataAsBase64();
+      entry.structuredCloneVersion = shEntry.stateData.formatVersion;
+    }
+
+    if (!(shEntry instanceof Ci.nsISHContainer)) {
+      return entry;
+    }
+
+    if (shEntry.childCount > 0 && !shEntry.hasDynamicallyAddedChild()) {
+      let children = [];
+      for (let i = 0; i < shEntry.childCount; i++) {
+        let child = shEntry.GetChildAt(i);
+
+        if (child) {
+          // Don't try to restore framesets containing wyciwyg URLs.
+          // (cf. bug 424689 and bug 450595)
+          if (child.URI.schemeIs("wyciwyg")) {
+            children.length = 0;
+            break;
+          }
+
+          children.push(this.serializeEntry(child));
+        }
+      }
+
+      if (children.length) {
+        entry.children = children;
+      }
+    }
+
+    return entry;
+  },
+
+  /**
+   * Restores session history data for a given docShell.
+   *
+   * @param docShell
+   *        The docShell that owns the session history.
+   * @param tabData
+   *        The tabdata including all history entries.
+   */
+  restore: function (docShell, tabData) {
+    let webNavigation = docShell.QueryInterface(Ci.nsIWebNavigation);
+    let history = webNavigation.sessionHistory;
+    if (history.count > 0) {
+      history.PurgeHistory(history.count);
+    }
+    history.QueryInterface(Ci.nsISHistoryInternal);
+
+    let idMap = { used: {} };
+    let docIdentMap = {};
+    for (let i = 0; i < tabData.entries.length; i++) {
+      let entry = tabData.entries[i];
+      //XXXzpao Wallpaper patch for bug 514751
+      if (!entry.url)
+        continue;
+      let persist = "persist" in entry ? entry.persist : true;
+      history.addEntry(this.deserializeEntry(entry, idMap, docIdentMap), persist);
+    }
+
+    // Select the right history entry.
+    let index = tabData.index - 1;
+    if (index < history.count && history.index != index) {
+      history.getEntryAtIndex(index, true);
+    }
+  },
+
+  /**
+   * Expands serialized history data into a session-history-entry instance.
+   *
+   * @param entry
+   *        Object containing serialized history data for a URL
+   * @param idMap
+   *        Hash for ensuring unique frame IDs
+   * @param docIdentMap
+   *        Hash to ensure reuse of BFCache entries
+   * @returns nsISHEntry
+   */
+  deserializeEntry: function (entry, idMap, docIdentMap) {
+
+    var shEntry = Cc["@mozilla.org/browser/session-history-entry;1"].
+                  createInstance(Ci.nsISHEntry);
+
+    shEntry.setURI(Utils.makeURI(entry.url, entry.charset));
+    shEntry.setTitle(entry.title || entry.url);
+    if (entry.subframe)
+      shEntry.setIsSubFrame(entry.subframe || false);
+    shEntry.loadType = Ci.nsIDocShellLoadInfo.loadHistory;
+    if (entry.contentType)
+      shEntry.contentType = entry.contentType;
+    if (entry.referrer) {
+      shEntry.referrerURI = Utils.makeURI(entry.referrer);
+      shEntry.referrerPolicy = entry.referrerPolicy;
+    }
+    if (entry.originalURI) {
+      shEntry.originalURI = Utils.makeURI(entry.originalURI);
+    }
+    if (entry.loadReplace) {
+      shEntry.loadReplace = entry.loadReplace;
+    }
+    if (entry.isSrcdocEntry)
+      shEntry.srcdocData = entry.srcdocData;
+    if (entry.baseURI)
+      shEntry.baseURI = Utils.makeURI(entry.baseURI);
+
+    if (entry.cacheKey) {
+      var cacheKey = Cc["@mozilla.org/supports-PRUint32;1"].
+                     createInstance(Ci.nsISupportsPRUint32);
+      cacheKey.data = entry.cacheKey;
+      shEntry.cacheKey = cacheKey;
+    }
+
+    if (entry.ID) {
+      // get a new unique ID for this frame (since the one from the last
+      // start might already be in use)
+      var id = idMap[entry.ID] || 0;
+      if (!id) {
+        for (id = Date.now(); id in idMap.used; id++);
+        idMap[entry.ID] = id;
+        idMap.used[id] = true;
+      }
+      shEntry.ID = id;
+    }
+
+    if (entry.docshellID)
+      shEntry.docshellID = entry.docshellID;
+
+    if (entry.structuredCloneState && entry.structuredCloneVersion) {
+      shEntry.stateData =
+        Cc["@mozilla.org/docshell/structured-clone-container;1"].
+        createInstance(Ci.nsIStructuredCloneContainer);
+
+      shEntry.stateData.initFromBase64(entry.structuredCloneState,
+                                       entry.structuredCloneVersion);
+    }
+
+    if (entry.scrollRestorationIsManual) {
+      shEntry.scrollRestorationIsManual = true;
+    } else if (entry.scroll) {
+      var scrollPos = (entry.scroll || "0,0").split(",");
+      scrollPos = [parseInt(scrollPos[0]) || 0, parseInt(scrollPos[1]) || 0];
+      shEntry.setScrollPosition(scrollPos[0], scrollPos[1]);
+    }
+
+    let childDocIdents = {};
+    if (entry.docIdentifier) {
+      // If we have a serialized document identifier, try to find an SHEntry
+      // which matches that doc identifier and adopt that SHEntry's
+      // BFCacheEntry.  If we don't find a match, insert shEntry as the match
+      // for the document identifier.
+      let matchingEntry = docIdentMap[entry.docIdentifier];
+      if (!matchingEntry) {
+        matchingEntry = {shEntry: shEntry, childDocIdents: childDocIdents};
+        docIdentMap[entry.docIdentifier] = matchingEntry;
+      }
+      else {
+        shEntry.adoptBFCacheEntry(matchingEntry.shEntry);
+        childDocIdents = matchingEntry.childDocIdents;
+      }
+    }
+
+    // The field entry.owner_b64 got renamed to entry.triggeringPricipal_b64 in
+    // Bug 1286472. To remain backward compatible we still have to support that
+    // field for a few cycles before we can remove it within Bug 1289785.
+    if (entry.owner_b64) {
+      entry.triggeringPricipal_b64 = entry.owner_b64;
+      delete entry.owner_b64;
+    }
+
+    // Before introducing the concept of principalToInherit we only had
+    // a triggeringPrincipal within every entry which basically is the
+    // equivalent of the new principalToInherit. To avoid compatibility
+    // issues, we first check if the entry has entries for
+    // triggeringPrincipal_base64 and principalToInherit_base64. If not
+    // we fall back to using the principalToInherit (which is stored
+    // as triggeringPrincipal_b64) as the triggeringPrincipal and
+    // the principalToInherit.
+    // FF55 will remove the triggeringPrincipal_b64, see Bug 1301666.
+    if (entry.triggeringPrincipal_base64 || entry.principalToInherit_base64) {
+      if (entry.triggeringPrincipal_base64) {
+        shEntry.triggeringPrincipal =
+          Utils.deserializePrincipal(entry.triggeringPrincipal_base64);
+      }
+      if (entry.principalToInherit_base64) {
+        shEntry.principalToInherit =
+          Utils.deserializePrincipal(entry.principalToInherit_base64);
+      }
+    } else if (entry.triggeringPrincipal_b64) {
+      shEntry.triggeringPrincipal = Utils.deserializePrincipal(entry.triggeringPrincipal_b64);
+      shEntry.principalToInherit = shEntry.triggeringPrincipal;
+    }
+
+    if (entry.children && shEntry instanceof Ci.nsISHContainer) {
+      for (var i = 0; i < entry.children.length; i++) {
+        //XXXzpao Wallpaper patch for bug 514751
+        if (!entry.children[i].url)
+          continue;
+
+        // We're getting sessionrestore.js files with a cycle in the
+        // doc-identifier graph, likely due to bug 698656.  (That is, we have
+        // an entry where doc identifier A is an ancestor of doc identifier B,
+        // and another entry where doc identifier B is an ancestor of A.)
+        //
+        // If we were to respect these doc identifiers, we'd create a cycle in
+        // the SHEntries themselves, which causes the docshell to loop forever
+        // when it looks for the root SHEntry.
+        //
+        // So as a hack to fix this, we restrict the scope of a doc identifier
+        // to be a node's siblings and cousins, and pass childDocIdents, not
+        // aDocIdents, to _deserializeHistoryEntry.  That is, we say that two
+        // SHEntries with the same doc identifier have the same document iff
+        // they have the same parent or their parents have the same document.
+
+        shEntry.AddChild(this.deserializeEntry(entry.children[i], idMap,
+                                               childDocIdents), i);
+      }
+    }
+
+    return shEntry;
+  },
+
+};
diff --git a/application/basilisk/components/sessionstore/SessionMigration.jsm b/application/basilisk/components/sessionstore/SessionMigration.jsm
new file mode 100644
index 0000000000..1aa22f1a9d
--- /dev/null
+++ b/application/basilisk/components/sessionstore/SessionMigration.jsm
@@ -0,0 +1,106 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this file,
+ * You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+"use strict";
+
+this.EXPORTED_SYMBOLS = ["SessionMigration"];
+
+const Cu = Components.utils;
+Cu.import("resource://gre/modules/XPCOMUtils.jsm", this);
+Cu.import("resource://gre/modules/Task.jsm", this);
+Cu.import("resource://gre/modules/osfile.jsm", this);
+
+XPCOMUtils.defineLazyModuleGetter(this, "Utils",
+  "resource://gre/modules/sessionstore/Utils.jsm");
+
+// An encoder to UTF-8.
+XPCOMUtils.defineLazyGetter(this, "gEncoder", function () {
+  return new TextEncoder();
+});
+
+// A decoder.
+XPCOMUtils.defineLazyGetter(this, "gDecoder", function () {
+  return new TextDecoder();
+});
+
+var SessionMigrationInternal = {
+  /**
+   * Convert the original session restore state into a minimal state. It will
+   * only contain:
+   * - open windows
+   *   - with tabs
+   *     - with history entries with only title, url, triggeringPrincipal
+   *     - with pinned state
+   *     - with tab group info (hidden + group id)
+   *     - with selected tab info
+   *   - with selected window info
+   *
+   * The complete state is then wrapped into the "about:welcomeback" page as
+   * form field info to be restored when restoring the state.
+   */
+  convertState: function(aStateObj) {
+    let state = {
+      selectedWindow: aStateObj.selectedWindow,
+      _closedWindows: []
+    };
+    state.windows = aStateObj.windows.map(function(oldWin) {
+      var win = {extData: {}};
+      win.tabs = oldWin.tabs.map(function(oldTab) {
+        var tab = {};
+        // Keep only titles, urls and triggeringPrincipals for history entries
+        tab.entries = oldTab.entries.map(function(entry) {
+          return { url: entry.url,
+                   triggeringPrincipal_base64: entry.triggeringPrincipal_base64,
+                   title: entry.title };
+        });
+        tab.index = oldTab.index;
+        tab.hidden = oldTab.hidden;
+        tab.pinned = oldTab.pinned;
+        return tab;
+      });
+      win.selected = oldWin.selected;
+      win._closedTabs = [];
+      return win;
+    });
+    let url = "about:welcomeback";
+    let formdata = {id: {sessionData: state}, url};
+    let entry = { url, triggeringPrincipal_base64: Utils.SERIALIZED_SYSTEMPRINCIPAL };
+    return { windows: [{ tabs: [{ entries: [ entry ], formdata}]}]};
+  },
+  /**
+   * Asynchronously read session restore state (JSON) from a path
+   */
+  readState: function(aPath) {
+    return Task.spawn(function() {
+      let bytes = yield OS.File.read(aPath);
+      let text = gDecoder.decode(bytes);
+      let state = JSON.parse(text);
+      throw new Task.Result(state);
+    });
+  },
+  /**
+   * Asynchronously write session restore state as JSON to a path
+   */
+  writeState: function(aPath, aState) {
+    let bytes = gEncoder.encode(JSON.stringify(aState));
+    return OS.File.writeAtomic(aPath, bytes, {tmpPath: aPath + ".tmp"});
+  }
+}
+
+var SessionMigration = {
+  /**
+   * Migrate a limited set of session data from one path to another.
+   */
+  migrate: function(aFromPath, aToPath) {
+    return Task.spawn(function() {
+      let inState = yield SessionMigrationInternal.readState(aFromPath);
+      let outState = SessionMigrationInternal.convertState(inState);
+      // Unfortunately, we can't use SessionStore's own SessionFile to
+      // write out the data because it has a dependency on the profile dir
+      // being known. When the migration runs, there is no guarantee that
+      // that's true.
+      yield SessionMigrationInternal.writeState(aToPath, outState);
+    });
+  }
+};
diff --git a/application/basilisk/components/sessionstore/SessionSaver.jsm b/application/basilisk/components/sessionstore/SessionSaver.jsm
new file mode 100644
index 0000000000..d672f88774
--- /dev/null
+++ b/application/basilisk/components/sessionstore/SessionSaver.jsm
@@ -0,0 +1,264 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this file,
+ * You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+"use strict";
+
+this.EXPORTED_SYMBOLS = ["SessionSaver"];
+
+const Cu = Components.utils;
+const Cc = Components.classes;
+const Ci = Components.interfaces;
+
+Cu.import("resource://gre/modules/Timer.jsm", this);
+Cu.import("resource://gre/modules/Services.jsm", this);
+Cu.import("resource://gre/modules/XPCOMUtils.jsm", this);
+Cu.import("resource://gre/modules/TelemetryStopwatch.jsm", this);
+
+XPCOMUtils.defineLazyModuleGetter(this, "AppConstants",
+  "resource://gre/modules/AppConstants.jsm");
+XPCOMUtils.defineLazyModuleGetter(this, "console",
+  "resource://gre/modules/Console.jsm");
+XPCOMUtils.defineLazyModuleGetter(this, "PrivacyFilter",
+  "resource:///modules/sessionstore/PrivacyFilter.jsm");
+XPCOMUtils.defineLazyModuleGetter(this, "RunState",
+  "resource:///modules/sessionstore/RunState.jsm");
+XPCOMUtils.defineLazyModuleGetter(this, "SessionStore",
+  "resource:///modules/sessionstore/SessionStore.jsm");
+XPCOMUtils.defineLazyModuleGetter(this, "SessionFile",
+  "resource:///modules/sessionstore/SessionFile.jsm");
+XPCOMUtils.defineLazyModuleGetter(this, "PrivateBrowsingUtils",
+  "resource://gre/modules/PrivateBrowsingUtils.jsm");
+
+// Minimal interval between two save operations (in milliseconds).
+XPCOMUtils.defineLazyGetter(this, "gInterval", function () {
+  const PREF = "browser.sessionstore.interval";
+
+  // Observer that updates the cached value when the preference changes.
+  Services.prefs.addObserver(PREF, () => {
+    this.gInterval = Services.prefs.getIntPref(PREF);
+
+    // Cancel any pending runs and call runDelayed() with
+    // zero to apply the newly configured interval.
+    SessionSaverInternal.cancel();
+    SessionSaverInternal.runDelayed(0);
+  }, false);
+
+  return Services.prefs.getIntPref(PREF);
+});
+
+// Notify observers about a given topic with a given subject.
+function notify(subject, topic) {
+  Services.obs.notifyObservers(subject, topic, "");
+}
+
+// TelemetryStopwatch helper functions.
+function stopWatch(method) {
+  return function (...histograms) {
+    for (let hist of histograms) {
+      TelemetryStopwatch[method]("FX_SESSION_RESTORE_" + hist);
+    }
+  };
+}
+
+var stopWatchStart = stopWatch("start");
+var stopWatchCancel = stopWatch("cancel");
+var stopWatchFinish = stopWatch("finish");
+
+/**
+ * The external API implemented by the SessionSaver module.
+ */
+this.SessionSaver = Object.freeze({
+  /**
+   * Immediately saves the current session to disk.
+   */
+  run: function () {
+    return SessionSaverInternal.run();
+  },
+
+  /**
+   * Saves the current session to disk delayed by a given amount of time. Should
+   * another delayed run be scheduled already, we will ignore the given delay
+   * and state saving may occur a little earlier.
+   */
+  runDelayed: function () {
+    SessionSaverInternal.runDelayed();
+  },
+
+  /**
+   * Sets the last save time to the current time. This will cause us to wait for
+   * at least the configured interval when runDelayed() is called next.
+   */
+  updateLastSaveTime: function () {
+    SessionSaverInternal.updateLastSaveTime();
+  },
+
+  /**
+   * Cancels all pending session saves.
+   */
+  cancel: function () {
+    SessionSaverInternal.cancel();
+  }
+});
+
+/**
+ * The internal API.
+ */
+var SessionSaverInternal = {
+  /**
+   * The timeout ID referencing an active timer for a delayed save. When no
+   * save is pending, this is null.
+   */
+  _timeoutID: null,
+
+  /**
+   * A timestamp that keeps track of when we saved the session last. We will
+   * this to determine the correct interval between delayed saves to not deceed
+   * the configured session write interval.
+   */
+  _lastSaveTime: 0,
+
+  /**
+   * Immediately saves the current session to disk.
+   */
+  run: function () {
+    return this._saveState(true /* force-update all windows */);
+  },
+
+  /**
+   * Saves the current session to disk delayed by a given amount of time. Should
+   * another delayed run be scheduled already, we will ignore the given delay
+   * and state saving may occur a little earlier.
+   *
+   * @param delay (optional)
+   *        The minimum delay in milliseconds to wait for until we collect and
+   *        save the current session.
+   */
+  runDelayed: function (delay = 2000) {
+    // Bail out if there's a pending run.
+    if (this._timeoutID) {
+      return;
+    }
+
+    // Interval until the next disk operation is allowed.
+    delay = Math.max(this._lastSaveTime + gInterval - Date.now(), delay, 0);
+
+    // Schedule a state save.
+    this._timeoutID = setTimeout(() => this._saveStateAsync(), delay);
+  },
+
+  /**
+   * Sets the last save time to the current time. This will cause us to wait for
+   * at least the configured interval when runDelayed() is called next.
+   */
+  updateLastSaveTime: function () {
+    this._lastSaveTime = Date.now();
+  },
+
+  /**
+   * Cancels all pending session saves.
+   */
+  cancel: function () {
+    clearTimeout(this._timeoutID);
+    this._timeoutID = null;
+  },
+
+  /**
+   * Saves the current session state. Collects data and writes to disk.
+   *
+   * @param forceUpdateAllWindows (optional)
+   *        Forces us to recollect data for all windows and will bypass and
+   *        update the corresponding caches.
+   */
+  _saveState: function (forceUpdateAllWindows = false) {
+    // Cancel any pending timeouts.
+    this.cancel();
+
+    if (PrivateBrowsingUtils.permanentPrivateBrowsing) {
+      // Don't save (or even collect) anything in permanent private
+      // browsing mode
+
+      this.updateLastSaveTime();
+      return Promise.resolve();
+    }
+
+    stopWatchStart("COLLECT_DATA_MS", "COLLECT_DATA_LONGEST_OP_MS");
+    let state = SessionStore.getCurrentState(forceUpdateAllWindows);
+    PrivacyFilter.filterPrivateWindowsAndTabs(state);
+
+    // Make sure that we keep the previous session if we started with a single
+    // private window and no non-private windows have been opened, yet.
+    if (state.deferredInitialState) {
+      state.windows = state.deferredInitialState.windows || [];
+      delete state.deferredInitialState;
+    }
+
+    if (AppConstants.platform != "macosx") {
+      // We want to restore closed windows that are marked with _shouldRestore.
+      // We're doing this here because we want to control this only when saving
+      // the file.
+      while (state._closedWindows.length) {
+        let i = state._closedWindows.length - 1;
+
+        if (!state._closedWindows[i]._shouldRestore) {
+          // We only need to go until _shouldRestore
+          // is falsy since we're going in reverse.
+          break;
+        }
+
+        delete state._closedWindows[i]._shouldRestore;
+        state.windows.unshift(state._closedWindows.pop());
+      }
+    }
+
+    // Clear all cookies on clean shutdown according to user preferences
+    if (RunState.isClosing) {
+      let expireCookies = Services.prefs.getIntPref("network.cookie.lifetimePolicy") ==
+                          Services.cookies.QueryInterface(Ci.nsICookieService).ACCEPT_SESSION;
+      let sanitizeCookies = Services.prefs.getBoolPref("privacy.sanitize.sanitizeOnShutdown") &&
+                            Services.prefs.getBoolPref("privacy.clearOnShutdown.cookies");
+      let restart = Services.prefs.getBoolPref("browser.sessionstore.resume_session_once");
+      // Don't clear cookies when restarting
+      if ((expireCookies || sanitizeCookies) && !restart) {
+        for (let window of state.windows) {
+          delete window.cookies;
+        }
+      }
+    }
+
+    stopWatchFinish("COLLECT_DATA_MS", "COLLECT_DATA_LONGEST_OP_MS");
+    return this._writeState(state);
+  },
+
+  /**
+   * Saves the current session state. Collects data asynchronously and calls
+   * _saveState() to collect data again (with a cache hit rate of hopefully
+   * 100%) and write to disk afterwards.
+   */
+  _saveStateAsync: function () {
+    // Allow scheduling delayed saves again.
+    this._timeoutID = null;
+
+    // Write to disk.
+    this._saveState();
+  },
+
+  /**
+   * Write the given state object to disk.
+   */
+  _writeState: function (state) {
+    // We update the time stamp before writing so that we don't write again
+    // too soon, if saving is requested before the write completes. Without
+    // this update we may save repeatedly if actions cause a runDelayed
+    // before writing has completed. See Bug 902280
+    this.updateLastSaveTime();
+
+    // Write (atomically) to a session file, using a tmp file. Once the session
+    // file is successfully updated, save the time stamp of the last save and
+    // notify the observers.
+    return SessionFile.write(state).then(() => {
+      this.updateLastSaveTime();
+      notify(null, "sessionstore-state-write-complete");
+    }, console.error);
+  },
+};
diff --git a/application/basilisk/components/sessionstore/SessionStorage.jsm b/application/basilisk/components/sessionstore/SessionStorage.jsm
new file mode 100644
index 0000000000..705139ebfd
--- /dev/null
+++ b/application/basilisk/components/sessionstore/SessionStorage.jsm
@@ -0,0 +1,173 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+* License, v. 2.0. If a copy of the MPL was not distributed with this file,
+* You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+"use strict";
+
+this.EXPORTED_SYMBOLS = ["SessionStorage"];
+
+const Cu = Components.utils;
+const Ci = Components.interfaces;
+
+Cu.import("resource://gre/modules/Services.jsm");
+Cu.import("resource://gre/modules/XPCOMUtils.jsm");
+
+XPCOMUtils.defineLazyModuleGetter(this, "console",
+  "resource://gre/modules/Console.jsm");
+
+// Returns the principal for a given |frame| contained in a given |docShell|.
+function getPrincipalForFrame(docShell, frame) {
+  let ssm = Services.scriptSecurityManager;
+  let uri = frame.document.documentURIObject;
+  return ssm.getDocShellCodebasePrincipal(uri, docShell);
+}
+
+this.SessionStorage = Object.freeze({
+  /**
+   * Updates all sessionStorage "super cookies"
+   * @param docShell
+   *        That tab's docshell (containing the sessionStorage)
+   * @param frameTree
+   *        The docShell's FrameTree instance.
+   * @return Returns a nested object that will have hosts as keys and per-host
+   *         session storage data as strings. For example:
+   *         {"example.com": {"key": "value", "my_number": "123"}}
+   */
+  collect: function (docShell, frameTree) {
+    return SessionStorageInternal.collect(docShell, frameTree);
+  },
+
+  /**
+   * Restores all sessionStorage "super cookies".
+   * @param aDocShell
+   *        A tab's docshell (containing the sessionStorage)
+   * @param aStorageData
+   *        A nested object with storage data to be restored that has hosts as
+   *        keys and per-host session storage data as strings. For example:
+   *        {"example.com": {"key": "value", "my_number": "123"}}
+   */
+  restore: function (aDocShell, aStorageData) {
+    SessionStorageInternal.restore(aDocShell, aStorageData);
+  },
+});
+
+var SessionStorageInternal = {
+  /**
+   * Reads all session storage data from the given docShell.
+   * @param docShell
+   *        A tab's docshell (containing the sessionStorage)
+   * @param frameTree
+   *        The docShell's FrameTree instance.
+   * @return Returns a nested object that will have hosts as keys and per-host
+   *         session storage data as strings. For example:
+   *         {"example.com": {"key": "value", "my_number": "123"}}
+   */
+  collect: function (docShell, frameTree) {
+    let data = {};
+    let visitedOrigins = new Set();
+
+    frameTree.forEach(frame => {
+      let principal = getPrincipalForFrame(docShell, frame);
+      if (!principal) {
+        return;
+      }
+
+      // Get the origin of the current history entry
+      // and use that as a key for the per-principal storage data.
+      let origin = principal.origin;
+      if (visitedOrigins.has(origin)) {
+        // Don't read a host twice.
+        return;
+      }
+
+      // Mark the current origin as visited.
+      visitedOrigins.add(origin);
+
+      let originData = this._readEntry(principal, docShell);
+      if (Object.keys(originData).length) {
+        data[origin] = originData;
+      }
+    });
+
+    return Object.keys(data).length ? data : null;
+  },
+
+  /**
+   * Writes session storage data to the given tab.
+   * @param aDocShell
+   *        A tab's docshell (containing the sessionStorage)
+   * @param aStorageData
+   *        A nested object with storage data to be restored that has hosts as
+   *        keys and per-host session storage data as strings. For example:
+   *        {"example.com": {"key": "value", "my_number": "123"}}
+   */
+  restore: function (aDocShell, aStorageData) {
+    for (let origin of Object.keys(aStorageData)) {
+      let data = aStorageData[origin];
+
+      let principal;
+
+      try {
+        let attrs = aDocShell.getOriginAttributes();
+        let originURI = Services.io.newURI(origin, null, null);
+        principal = Services.scriptSecurityManager.createCodebasePrincipal(originURI, attrs);
+      } catch (e) {
+        console.error(e);
+        continue;
+      }
+
+      let storageManager = aDocShell.QueryInterface(Ci.nsIDOMStorageManager);
+      let window = aDocShell.QueryInterface(Ci.nsIInterfaceRequestor).getInterface(Ci.nsIDOMWindow);
+
+      // There is no need to pass documentURI, it's only used to fill documentURI property of
+      // domstorage event, which in this case has no consumer. Prevention of events in case
+      // of missing documentURI will be solved in a followup bug to bug 600307.
+      let storage = storageManager.createStorage(window, principal, "", aDocShell.usePrivateBrowsing);
+
+      for (let key of Object.keys(data)) {
+        try {
+          storage.setItem(key, data[key]);
+        } catch (e) {
+          // throws e.g. for URIs that can't have sessionStorage
+          console.error(e);
+        }
+      }
+    }
+  },
+
+  /**
+   * Reads an entry in the session storage data contained in a tab's history.
+   * @param aURI
+   *        That history entry uri
+   * @param aDocShell
+   *        A tab's docshell (containing the sessionStorage)
+   */
+  _readEntry: function (aPrincipal, aDocShell) {
+    let hostData = {};
+    let storage;
+
+    let window = aDocShell.QueryInterface(Ci.nsIInterfaceRequestor).getInterface(Ci.nsIDOMWindow);
+
+    try {
+      let storageManager = aDocShell.QueryInterface(Ci.nsIDOMStorageManager);
+      storage = storageManager.getStorage(window, aPrincipal);
+      storage.length; // XXX: Bug 1232955 - storage.length can throw, catch that failure
+    } catch (e) {
+      // sessionStorage might throw if it's turned off, see bug 458954
+      storage = null;
+    }
+
+    if (storage && storage.length) {
+       for (let i = 0; i < storage.length; i++) {
+        try {
+          let key = storage.key(i);
+          hostData[key] = storage.getItem(key);
+        } catch (e) {
+          // This currently throws for secured items (cf. bug 442048).
+        }
+      }
+    }
+
+    return hostData;
+  }
+};
diff --git a/application/basilisk/components/sessionstore/SessionStore.jsm b/application/basilisk/components/sessionstore/SessionStore.jsm
new file mode 100644
index 0000000000..6b30943f38
--- /dev/null
+++ b/application/basilisk/components/sessionstore/SessionStore.jsm
@@ -0,0 +1,4746 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this file,
+ * You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+"use strict";
+
+this.EXPORTED_SYMBOLS = ["SessionStore"];
+
+const Cu = Components.utils;
+const Cc = Components.classes;
+const Ci = Components.interfaces;
+const Cr = Components.results;
+
+// Current version of the format used by Session Restore.
+const FORMAT_VERSION = 1;
+
+const TAB_STATE_NEEDS_RESTORE = 1;
+const TAB_STATE_RESTORING = 2;
+const TAB_STATE_WILL_RESTORE = 3;
+
+// A new window has just been restored. At this stage, tabs are generally
+// not restored.
+const NOTIFY_SINGLE_WINDOW_RESTORED = "sessionstore-single-window-restored";
+const NOTIFY_WINDOWS_RESTORED = "sessionstore-windows-restored";
+const NOTIFY_BROWSER_STATE_RESTORED = "sessionstore-browser-state-restored";
+const NOTIFY_LAST_SESSION_CLEARED = "sessionstore-last-session-cleared";
+const NOTIFY_RESTORING_ON_STARTUP = "sessionstore-restoring-on-startup";
+const NOTIFY_INITIATING_MANUAL_RESTORE = "sessionstore-initiating-manual-restore";
+
+const NOTIFY_TAB_RESTORED = "sessionstore-debug-tab-restored"; // WARNING: debug-only
+
+// Maximum number of tabs to restore simultaneously. Previously controlled by
+// the browser.sessionstore.max_concurrent_tabs pref.
+const MAX_CONCURRENT_TAB_RESTORES = 3;
+
+// Amount (in CSS px) by which we allow window edges to be off-screen
+// when restoring a window, before we override the saved position to
+// pull the window back within the available screen area.
+const SCREEN_EDGE_SLOP = 8;
+
+// global notifications observed
+const OBSERVING = [
+  "browser-window-before-show", "domwindowclosed",
+  "quit-application-granted", "browser-lastwindow-close-granted",
+  "quit-application", "browser:purge-session-history",
+  "browser:purge-domain-data",
+  "idle-daily",
+];
+
+// XUL Window properties to (re)store
+// Restored in restoreDimensions()
+const WINDOW_ATTRIBUTES = ["width", "height", "screenX", "screenY", "sizemode"];
+
+// Hideable window features to (re)store
+// Restored in restoreWindowFeatures()
+const WINDOW_HIDEABLE_FEATURES = [
+  "menubar", "toolbar", "locationbar", "personalbar", "statusbar", "scrollbars"
+];
+
+// Messages that will be received via the Frame Message Manager.
+const MESSAGES = [
+  // The content script sends us data that has been invalidated and needs to
+  // be saved to disk.
+  "SessionStore:update",
+
+  // The restoreHistory code has run. This is a good time to run SSTabRestoring.
+  "SessionStore:restoreHistoryComplete",
+
+  // The load for the restoring tab has begun. We update the URL bar at this
+  // time; if we did it before, the load would overwrite it.
+  "SessionStore:restoreTabContentStarted",
+
+  // All network loads for a restoring tab are done, so we should
+  // consider restoring another tab in the queue. The document has
+  // been restored, and forms have been filled. We trigger
+  // SSTabRestored at this time.
+  "SessionStore:restoreTabContentComplete",
+
+  // A crashed tab was revived by navigating to a different page. Remove its
+  // browser from the list of crashed browsers to stop ignoring its messages.
+  "SessionStore:crashedTabRevived",
+
+  // The content script encountered an error.
+  "SessionStore:error",
+];
+
+// The list of messages we accept from <xul:browser>s that have no tab
+// assigned, or whose windows have gone away. Those are for example the
+// ones that preload about:newtab pages, or from browsers where the window
+// has just been closed.
+const NOTAB_MESSAGES = new Set([
+  // For a description see above.
+  "SessionStore:crashedTabRevived",
+
+  // For a description see above.
+  "SessionStore:update",
+
+  // For a description see above.
+  "SessionStore:error",
+]);
+
+// The list of messages we accept without an "epoch" parameter.
+// See getCurrentEpoch() and friends to find out what an "epoch" is.
+const NOEPOCH_MESSAGES = new Set([
+  // For a description see above.
+  "SessionStore:crashedTabRevived",
+
+  // For a description see above.
+  "SessionStore:error",
+]);
+
+// The list of messages we want to receive even during the short period after a
+// frame has been removed from the DOM and before its frame script has finished
+// unloading.
+const CLOSED_MESSAGES = new Set([
+  // For a description see above.
+  "SessionStore:crashedTabRevived",
+
+  // For a description see above.
+  "SessionStore:update",
+
+  // For a description see above.
+  "SessionStore:error",
+]);
+
+// These are tab events that we listen to.
+const TAB_EVENTS = [
+  "TabOpen", "TabBrowserInserted", "TabClose", "TabSelect", "TabShow", "TabHide", "TabPinned",
+  "TabUnpinned"
+];
+
+const NS_XUL = "http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul";
+
+Cu.import("resource://gre/modules/PrivateBrowsingUtils.jsm", this);
+Cu.import("resource://gre/modules/Promise.jsm", this);
+Cu.import("resource://gre/modules/Services.jsm", this);
+Cu.import("resource://gre/modules/Task.jsm", this);
+Cu.import("resource://gre/modules/TelemetryStopwatch.jsm", this);
+Cu.import("resource://gre/modules/TelemetryTimestamps.jsm", this);
+Cu.import("resource://gre/modules/Timer.jsm", this);
+Cu.import("resource://gre/modules/XPCOMUtils.jsm", this);
+Cu.import("resource://gre/modules/debug.js", this);
+Cu.import("resource://gre/modules/osfile.jsm", this);
+
+XPCOMUtils.defineLazyServiceGetter(this, "gSessionStartup",
+  "@mozilla.org/browser/sessionstartup;1", "nsISessionStartup");
+XPCOMUtils.defineLazyServiceGetter(this, "gScreenManager",
+  "@mozilla.org/gfx/screenmanager;1", "nsIScreenManager");
+XPCOMUtils.defineLazyServiceGetter(this, "Telemetry",
+  "@mozilla.org/base/telemetry;1", "nsITelemetry");
+XPCOMUtils.defineLazyModuleGetter(this, "console",
+  "resource://gre/modules/Console.jsm");
+XPCOMUtils.defineLazyModuleGetter(this, "RecentWindow",
+  "resource:///modules/RecentWindow.jsm");
+
+XPCOMUtils.defineLazyModuleGetter(this, "AppConstants",
+  "resource://gre/modules/AppConstants.jsm");
+XPCOMUtils.defineLazyModuleGetter(this, "GlobalState",
+  "resource:///modules/sessionstore/GlobalState.jsm");
+XPCOMUtils.defineLazyModuleGetter(this, "PrivacyFilter",
+  "resource:///modules/sessionstore/PrivacyFilter.jsm");
+XPCOMUtils.defineLazyModuleGetter(this, "RunState",
+  "resource:///modules/sessionstore/RunState.jsm");
+XPCOMUtils.defineLazyModuleGetter(this, "ScratchpadManager",
+  "resource://devtools/client/scratchpad/scratchpad-manager.jsm");
+XPCOMUtils.defineLazyModuleGetter(this, "SessionSaver",
+  "resource:///modules/sessionstore/SessionSaver.jsm");
+XPCOMUtils.defineLazyModuleGetter(this, "SessionCookies",
+  "resource:///modules/sessionstore/SessionCookies.jsm");
+XPCOMUtils.defineLazyModuleGetter(this, "SessionFile",
+  "resource:///modules/sessionstore/SessionFile.jsm");
+XPCOMUtils.defineLazyModuleGetter(this, "TabAttributes",
+  "resource:///modules/sessionstore/TabAttributes.jsm");
+XPCOMUtils.defineLazyModuleGetter(this, "TabCrashHandler",
+  "resource:///modules/ContentCrashHandlers.jsm");
+XPCOMUtils.defineLazyModuleGetter(this, "TabState",
+  "resource:///modules/sessionstore/TabState.jsm");
+XPCOMUtils.defineLazyModuleGetter(this, "TabStateCache",
+  "resource:///modules/sessionstore/TabStateCache.jsm");
+XPCOMUtils.defineLazyModuleGetter(this, "TabStateFlusher",
+  "resource:///modules/sessionstore/TabStateFlusher.jsm");
+XPCOMUtils.defineLazyModuleGetter(this, "Utils",
+  "resource://gre/modules/sessionstore/Utils.jsm");
+XPCOMUtils.defineLazyModuleGetter(this, "ViewSourceBrowser",
+  "resource://gre/modules/ViewSourceBrowser.jsm");
+XPCOMUtils.defineLazyModuleGetter(this, "AsyncShutdown",
+  "resource://gre/modules/AsyncShutdown.jsm");
+
+Object.defineProperty(this, "HUDService", {
+  get: function HUDService_getter() {
+    let devtools = Cu.import("resource://devtools/shared/Loader.jsm", {}).devtools;
+    return devtools.require("devtools/client/webconsole/hudservice").HUDService;
+  },
+  configurable: true,
+  enumerable: true
+});
+
+/**
+ * |true| if we are in debug mode, |false| otherwise.
+ * Debug mode is controlled by preference browser.sessionstore.debug
+ */
+var gDebuggingEnabled = false;
+function debug(aMsg) {
+  if (gDebuggingEnabled) {
+    aMsg = ("SessionStore: " + aMsg).replace(/\S{80}/g, "$&\n");
+    Services.console.logStringMessage(aMsg);
+  }
+}
+
+this.SessionStore = {
+  get promiseInitialized() {
+    return SessionStoreInternal.promiseInitialized;
+  },
+
+  get canRestoreLastSession() {
+    return SessionStoreInternal.canRestoreLastSession;
+  },
+
+  set canRestoreLastSession(val) {
+    SessionStoreInternal.canRestoreLastSession = val;
+  },
+
+  get lastClosedObjectType() {
+    return SessionStoreInternal.lastClosedObjectType;
+  },
+
+  init: function ss_init() {
+    SessionStoreInternal.init();
+  },
+
+  getBrowserState: function ss_getBrowserState() {
+    return SessionStoreInternal.getBrowserState();
+  },
+
+  setBrowserState: function ss_setBrowserState(aState) {
+    SessionStoreInternal.setBrowserState(aState);
+  },
+
+  getWindowState: function ss_getWindowState(aWindow) {
+    return SessionStoreInternal.getWindowState(aWindow);
+  },
+
+  setWindowState: function ss_setWindowState(aWindow, aState, aOverwrite) {
+    SessionStoreInternal.setWindowState(aWindow, aState, aOverwrite);
+  },
+
+  getTabState: function ss_getTabState(aTab) {
+    return SessionStoreInternal.getTabState(aTab);
+  },
+
+  setTabState: function ss_setTabState(aTab, aState) {
+    SessionStoreInternal.setTabState(aTab, aState);
+  },
+
+  duplicateTab: function ss_duplicateTab(aWindow, aTab, aDelta = 0) {
+    return SessionStoreInternal.duplicateTab(aWindow, aTab, aDelta);
+  },
+
+  getClosedTabCount: function ss_getClosedTabCount(aWindow) {
+    return SessionStoreInternal.getClosedTabCount(aWindow);
+  },
+
+  getClosedTabData: function ss_getClosedTabData(aWindow, aAsString = true) {
+    return SessionStoreInternal.getClosedTabData(aWindow, aAsString);
+  },
+
+  undoCloseTab: function ss_undoCloseTab(aWindow, aIndex) {
+    return SessionStoreInternal.undoCloseTab(aWindow, aIndex);
+  },
+
+  forgetClosedTab: function ss_forgetClosedTab(aWindow, aIndex) {
+    return SessionStoreInternal.forgetClosedTab(aWindow, aIndex);
+  },
+
+  getClosedWindowCount: function ss_getClosedWindowCount() {
+    return SessionStoreInternal.getClosedWindowCount();
+  },
+
+  getClosedWindowData: function ss_getClosedWindowData(aAsString = true) {
+    return SessionStoreInternal.getClosedWindowData(aAsString);
+  },
+
+  undoCloseWindow: function ss_undoCloseWindow(aIndex) {
+    return SessionStoreInternal.undoCloseWindow(aIndex);
+  },
+
+  forgetClosedWindow: function ss_forgetClosedWindow(aIndex) {
+    return SessionStoreInternal.forgetClosedWindow(aIndex);
+  },
+
+  getWindowValue: function ss_getWindowValue(aWindow, aKey) {
+    return SessionStoreInternal.getWindowValue(aWindow, aKey);
+  },
+
+  setWindowValue: function ss_setWindowValue(aWindow, aKey, aStringValue) {
+    SessionStoreInternal.setWindowValue(aWindow, aKey, aStringValue);
+  },
+
+  deleteWindowValue: function ss_deleteWindowValue(aWindow, aKey) {
+    SessionStoreInternal.deleteWindowValue(aWindow, aKey);
+  },
+
+  getTabValue: function ss_getTabValue(aTab, aKey) {
+    return SessionStoreInternal.getTabValue(aTab, aKey);
+  },
+
+  setTabValue: function ss_setTabValue(aTab, aKey, aStringValue) {
+    SessionStoreInternal.setTabValue(aTab, aKey, aStringValue);
+  },
+
+  deleteTabValue: function ss_deleteTabValue(aTab, aKey) {
+    SessionStoreInternal.deleteTabValue(aTab, aKey);
+  },
+
+  getGlobalValue: function ss_getGlobalValue(aKey) {
+    return SessionStoreInternal.getGlobalValue(aKey);
+  },
+
+  setGlobalValue: function ss_setGlobalValue(aKey, aStringValue) {
+    SessionStoreInternal.setGlobalValue(aKey, aStringValue);
+  },
+
+  deleteGlobalValue: function ss_deleteGlobalValue(aKey) {
+    SessionStoreInternal.deleteGlobalValue(aKey);
+  },
+
+  persistTabAttribute: function ss_persistTabAttribute(aName) {
+    SessionStoreInternal.persistTabAttribute(aName);
+  },
+
+  restoreLastSession: function ss_restoreLastSession() {
+    SessionStoreInternal.restoreLastSession();
+  },
+
+  getCurrentState: function (aUpdateAll) {
+    return SessionStoreInternal.getCurrentState(aUpdateAll);
+  },
+
+  reviveCrashedTab(aTab) {
+    return SessionStoreInternal.reviveCrashedTab(aTab);
+  },
+
+  reviveAllCrashedTabs() {
+    return SessionStoreInternal.reviveAllCrashedTabs();
+  },
+
+  navigateAndRestore(tab, loadArguments, historyIndex) {
+    return SessionStoreInternal.navigateAndRestore(tab, loadArguments, historyIndex);
+  },
+
+  getSessionHistory(tab, updatedCallback) {
+    return SessionStoreInternal.getSessionHistory(tab, updatedCallback);
+  },
+
+  undoCloseById(aClosedId) {
+    return SessionStoreInternal.undoCloseById(aClosedId);
+  },
+
+  /**
+   * Determines whether the passed version number is compatible with
+   * the current version number of the SessionStore.
+   *
+   * @param version The format and version of the file, as an array, e.g.
+   * ["sessionrestore", 1]
+   */
+  isFormatVersionCompatible(version) {
+    if (!version) {
+      return false;
+    }
+    if (!Array.isArray(version)) {
+      // Improper format.
+      return false;
+    }
+    if (version[0] != "sessionrestore") {
+      // Not a Session Restore file.
+      return false;
+    }
+    let number = Number.parseFloat(version[1]);
+    if (Number.isNaN(number)) {
+      return false;
+    }
+    return number <= FORMAT_VERSION;
+  },
+};
+
+// Freeze the SessionStore object. We don't want anyone to modify it.
+Object.freeze(SessionStore);
+
+var SessionStoreInternal = {
+  QueryInterface: XPCOMUtils.generateQI([
+    Ci.nsIDOMEventListener,
+    Ci.nsIObserver,
+    Ci.nsISupportsWeakReference
+  ]),
+
+  _globalState: new GlobalState(),
+
+  // A counter to be used to generate a unique ID for each closed tab or window.
+  _nextClosedId: 0,
+
+  // During the initial restore and setBrowserState calls tracks the number of
+  // windows yet to be restored
+  _restoreCount: -1,
+
+  // For each <browser> element, records the current epoch.
+  _browserEpochs: new WeakMap(),
+
+  // Any browsers that fires the oop-browser-crashed event gets stored in
+  // here - that way we know which browsers to ignore messages from (until
+  // they get restored).
+  _crashedBrowsers: new WeakSet(),
+
+  // A map (xul:browser -> nsIFrameLoader) that maps a browser to the last
+  // associated frameLoader we heard about.
+  _lastKnownFrameLoader: new WeakMap(),
+
+  // A map (xul:browser -> object) that maps a browser associated with a
+  // recently closed tab to all its necessary state information we need to
+  // properly handle final update message.
+  _closedTabs: new WeakMap(),
+
+  // A map (xul:browser -> object) that maps a browser associated with a
+  // recently closed tab due to a window closure to the tab state information
+  // that is being stored in _closedWindows for that tab.
+  _closedWindowTabs: new WeakMap(),
+
+  // A set of window data that has the potential to be saved in the _closedWindows
+  // array for the session. We will remove window data from this set whenever
+  // forgetClosedWindow is called for the window, or when session history is
+  // purged, so that we don't accidentally save that data after the flush has
+  // completed. Closed tabs use a more complicated mechanism for this particular
+  // problem. When forgetClosedTab is called, the browser is removed from the
+  // _closedTabs map, so its data is not recorded. In the purge history case,
+  // the closedTabs array per window is overwritten so that once the flush is
+  // complete, the tab would only ever add itself to an array that SessionStore
+  // no longer cares about. Bug 1230636 has been filed to make the tab case
+  // work more like the window case, which is more explicit, and easier to
+  // reason about.
+  _saveableClosedWindowData: new WeakSet(),
+
+  // A map (xul:browser -> object) that maps a browser that is switching
+  // remoteness via navigateAndRestore, to the loadArguments that were
+  // most recently passed when calling navigateAndRestore.
+  _remotenessChangingBrowsers: new WeakMap(),
+
+  // whether a setBrowserState call is in progress
+  _browserSetState: false,
+
+  // time in milliseconds when the session was started (saved across sessions),
+  // defaults to now if no session was restored or timestamp doesn't exist
+  _sessionStartTime: Date.now(),
+
+  // states for all currently opened windows
+  _windows: {},
+
+  // counter for creating unique window IDs
+  _nextWindowID: 0,
+
+  // states for all recently closed windows
+  _closedWindows: [],
+
+  // collection of session states yet to be restored
+  _statesToRestore: {},
+
+  // counts the number of crashes since the last clean start
+  _recentCrashes: 0,
+
+  // whether the last window was closed and should be restored
+  _restoreLastWindow: false,
+
+  // number of tabs currently restoring
+  _tabsRestoringCount: 0,
+
+  // When starting Firefox with a single private window, this is the place
+  // where we keep the session we actually wanted to restore in case the user
+  // decides to later open a non-private window as well.
+  _deferredInitialState: null,
+
+  // A promise resolved once initialization is complete
+  _deferredInitialized: (function () {
+    let deferred = {};
+
+    deferred.promise = new Promise((resolve, reject) => {
+      deferred.resolve = resolve;
+      deferred.reject = reject;
+    });
+
+    return deferred;
+  })(),
+
+  // Whether session has been initialized
+  _sessionInitialized: false,
+
+  // Promise that is resolved when we're ready to initialize
+  // and restore the session.
+  _promiseReadyForInitialization: null,
+
+  // Keep busy state counters per window.
+  _windowBusyStates: new WeakMap(),
+
+  /**
+   * A promise fulfilled once initialization is complete.
+   */
+  get promiseInitialized() {
+    return this._deferredInitialized.promise;
+  },
+
+  get canRestoreLastSession() {
+    return LastSession.canRestore;
+  },
+
+  set canRestoreLastSession(val) {
+    // Cheat a bit; only allow false.
+    if (!val) {
+      LastSession.clear();
+    }
+  },
+
+  /**
+   * Returns a string describing the last closed object, either "tab" or "window".
+   *
+   * This was added to support the sessions.restore WebExtensions API.
+   */
+  get lastClosedObjectType() {
+    if (this._closedWindows.length) {
+      // Since there are closed windows, we need to check if there's a closed tab
+      // in one of the currently open windows that was closed after the
+      // last-closed window.
+      let tabTimestamps = [];
+      let windowsEnum = Services.wm.getEnumerator("navigator:browser");
+      while (windowsEnum.hasMoreElements()) {
+        let window = windowsEnum.getNext();
+        let windowState = this._windows[window.__SSi];
+        if (windowState && windowState._closedTabs[0]) {
+          tabTimestamps.push(windowState._closedTabs[0].closedAt);
+        }
+      }
+      if (!tabTimestamps.length ||
+          (tabTimestamps.sort((a, b) => b - a)[0] < this._closedWindows[0].closedAt)) {
+        return "window";
+      }
+    }
+    return "tab";
+  },
+
+  /**
+   * Initialize the sessionstore service.
+   */
+  init: function () {
+    if (this._initialized) {
+      throw new Error("SessionStore.init() must only be called once!");
+    }
+
+    TelemetryTimestamps.add("sessionRestoreInitialized");
+    OBSERVING.forEach(function(aTopic) {
+      Services.obs.addObserver(this, aTopic, true);
+    }, this);
+
+    this._initPrefs();
+    this._initialized = true;
+  },
+
+  /**
+   * Initialize the session using the state provided by SessionStartup
+   */
+  initSession: function () {
+    TelemetryStopwatch.start("FX_SESSION_RESTORE_STARTUP_INIT_SESSION_MS");
+    let state;
+    let ss = gSessionStartup;
+
+    if (ss.doRestore() ||
+        ss.sessionType == Ci.nsISessionStartup.DEFER_SESSION) {
+      state = ss.state;
+    }
+
+    if (state) {
+      try {
+        // If we're doing a DEFERRED session, then we want to pull pinned tabs
+        // out so they can be restored.
+        if (ss.sessionType == Ci.nsISessionStartup.DEFER_SESSION) {
+          let [iniState, remainingState] = this._prepDataForDeferredRestore(state);
+          // If we have a iniState with windows, that means that we have windows
+          // with app tabs to restore.
+          if (iniState.windows.length)
+            state = iniState;
+          else
+            state = null;
+
+          if (remainingState.windows.length) {
+            LastSession.setState(remainingState);
+          }
+        }
+        else {
+          // Get the last deferred session in case the user still wants to
+          // restore it
+          LastSession.setState(state.lastSessionState);
+
+          if (ss.previousSessionCrashed) {
+            this._recentCrashes = (state.session &&
+                                   state.session.recentCrashes || 0) + 1;
+
+            if (this._needsRestorePage(state, this._recentCrashes)) {
+              // replace the crashed session with a restore-page-only session
+              let url = "about:sessionrestore";
+              let formdata = {id: {sessionData: state}, url};
+              let entry = {url, triggeringPrincipal_base64: Utils.SERIALIZED_SYSTEMPRINCIPAL };
+              state = { windows: [{ tabs: [{ entries: [entry], formdata }] }] };
+            } else if (this._hasSingleTabWithURL(state.windows,
+                                                 "about:welcomeback")) {
+              // On a single about:welcomeback URL that crashed, replace about:welcomeback
+              // with about:sessionrestore, to make clear to the user that we crashed.
+              state.windows[0].tabs[0].entries[0].url = "about:sessionrestore";
+              state.windows[0].tabs[0].entries[0].triggeringPrincipal_base64 = Utils.SERIALIZED_SYSTEMPRINCIPAL;
+            }
+          }
+
+          // Update the session start time using the restored session state.
+          this._updateSessionStartTime(state);
+
+          // make sure that at least the first window doesn't have anything hidden
+          delete state.windows[0].hidden;
+          // Since nothing is hidden in the first window, it cannot be a popup
+          delete state.windows[0].isPopup;
+          // We don't want to minimize and then open a window at startup.
+          if (state.windows[0].sizemode == "minimized")
+            state.windows[0].sizemode = "normal";
+          // clear any lastSessionWindowID attributes since those don't matter
+          // during normal restore
+          state.windows.forEach(function(aWindow) {
+            delete aWindow.__lastSessionWindowID;
+          });
+        }
+      }
+      catch (ex) { debug("The session file is invalid: " + ex); }
+    }
+
+    // at this point, we've as good as resumed the session, so we can
+    // clear the resume_session_once flag, if it's set
+    if (!RunState.isQuitting &&
+        this._prefBranch.getBoolPref("sessionstore.resume_session_once"))
+      this._prefBranch.setBoolPref("sessionstore.resume_session_once", false);
+
+    TelemetryStopwatch.finish("FX_SESSION_RESTORE_STARTUP_INIT_SESSION_MS");
+    return state;
+  },
+
+  _initPrefs : function() {
+    this._prefBranch = Services.prefs.getBranch("browser.");
+
+    gDebuggingEnabled = this._prefBranch.getBoolPref("sessionstore.debug");
+
+    Services.prefs.addObserver("browser.sessionstore.debug", () => {
+      gDebuggingEnabled = this._prefBranch.getBoolPref("sessionstore.debug");
+    }, false);
+
+    this._max_tabs_undo = this._prefBranch.getIntPref("sessionstore.max_tabs_undo");
+    this._prefBranch.addObserver("sessionstore.max_tabs_undo", this, true);
+
+    this._max_windows_undo = this._prefBranch.getIntPref("sessionstore.max_windows_undo");
+    this._prefBranch.addObserver("sessionstore.max_windows_undo", this, true);
+  },
+
+  /**
+   * Called on application shutdown, after notifications:
+   * quit-application-granted, quit-application
+   */
+  _uninit: function ssi_uninit() {
+    if (!this._initialized) {
+      throw new Error("SessionStore is not initialized.");
+    }
+
+    // Prepare to close the session file and write the last state.
+    RunState.setClosing();
+
+    // save all data for session resuming
+    if (this._sessionInitialized) {
+      SessionSaver.run();
+    }
+
+    // clear out priority queue in case it's still holding refs
+    TabRestoreQueue.reset();
+
+    // Make sure to cancel pending saves.
+    SessionSaver.cancel();
+  },
+
+  /**
+   * Handle notifications
+   */
+  observe: function ssi_observe(aSubject, aTopic, aData) {
+    switch (aTopic) {
+      case "browser-window-before-show": // catch new windows
+        this.onBeforeBrowserWindowShown(aSubject);
+        break;
+      case "domwindowclosed": // catch closed windows
+        this.onClose(aSubject);
+        break;
+      case "quit-application-granted":
+        let syncShutdown = aData == "syncShutdown";
+        this.onQuitApplicationGranted(syncShutdown);
+        break;
+      case "browser-lastwindow-close-granted":
+        this.onLastWindowCloseGranted();
+        break;
+      case "quit-application":
+        this.onQuitApplication(aData);
+        break;
+      case "browser:purge-session-history": // catch sanitization
+        this.onPurgeSessionHistory();
+        break;
+      case "browser:purge-domain-data":
+        this.onPurgeDomainData(aData);
+        break;
+      case "nsPref:changed": // catch pref changes
+        this.onPrefChange(aData);
+        break;
+      case "idle-daily":
+        this.onIdleDaily();
+        break;
+    }
+  },
+
+  /**
+   * This method handles incoming messages sent by the session store content
+   * script via the Frame Message Manager or Parent Process Message Manager,
+   * and thus enables communication with OOP tabs.
+   */
+  receiveMessage(aMessage) {
+    // If we got here, that means we're dealing with a frame message
+    // manager message, so the target will be a <xul:browser>.
+    var browser = aMessage.target;
+    let win = browser.ownerGlobal;
+    let tab = win ? win.gBrowser.getTabForBrowser(browser) : null;
+
+    // Ensure we receive only specific messages from <xul:browser>s that
+    // have no tab or window assigned, e.g. the ones that preload
+    // about:newtab pages, or windows that have closed.
+    if (!tab && !NOTAB_MESSAGES.has(aMessage.name)) {
+      throw new Error(`received unexpected message '${aMessage.name}' ` +
+                      `from a browser that has no tab or window`);
+    }
+
+    let data = aMessage.data || {};
+    let hasEpoch = data.hasOwnProperty("epoch");
+
+    // Most messages sent by frame scripts require to pass an epoch.
+    if (!hasEpoch && !NOEPOCH_MESSAGES.has(aMessage.name)) {
+      throw new Error(`received message '${aMessage.name}' without an epoch`);
+    }
+
+    // Ignore messages from previous epochs.
+    if (hasEpoch && !this.isCurrentEpoch(browser, data.epoch)) {
+      return;
+    }
+
+    switch (aMessage.name) {
+      case "SessionStore:update":
+        // |browser.frameLoader| might be empty if the browser was already
+        // destroyed and its tab removed. In that case we still have the last
+        // frameLoader we know about to compare.
+        let frameLoader = browser.frameLoader ||
+                          this._lastKnownFrameLoader.get(browser.permanentKey);
+
+        // If the message isn't targeting the latest frameLoader discard it.
+        if (frameLoader != aMessage.targetFrameLoader) {
+          return;
+        }
+
+        if (aMessage.data.isFinal) {
+          // If this the final message we need to resolve all pending flush
+          // requests for the given browser as they might have been sent too
+          // late and will never respond. If they have been sent shortly after
+          // switching a browser's remoteness there isn't too much data to skip.
+          TabStateFlusher.resolveAll(browser);
+        } else if (aMessage.data.flushID) {
+          // This is an update kicked off by an async flush request. Notify the
+          // TabStateFlusher so that it can finish the request and notify its
+          // consumer that's waiting for the flush to be done.
+          TabStateFlusher.resolve(browser, aMessage.data.flushID);
+        }
+
+        // Ignore messages from <browser> elements that have crashed
+        // and not yet been revived.
+        if (this._crashedBrowsers.has(browser.permanentKey)) {
+          return;
+        }
+
+        // Record telemetry measurements done in the child and update the tab's
+        // cached state. Mark the window as dirty and trigger a delayed write.
+        this.recordTelemetry(aMessage.data.telemetry);
+        TabState.update(browser, aMessage.data);
+        this.saveStateDelayed(win);
+
+        // Handle any updates sent by the child after the tab was closed. This
+        // might be the final update as sent by the "unload" handler but also
+        // any async update message that was sent before the child unloaded.
+        if (this._closedTabs.has(browser.permanentKey)) {
+          let {closedTabs, tabData} = this._closedTabs.get(browser.permanentKey);
+
+          // Update the closed tab's state. This will be reflected in its
+          // window's list of closed tabs as that refers to the same object.
+          TabState.copyFromCache(browser, tabData.state);
+
+          // Is this the tab's final message?
+          if (aMessage.data.isFinal) {
+            // We expect no further updates.
+            this._closedTabs.delete(browser.permanentKey);
+            // The tab state no longer needs this reference.
+            delete tabData.permanentKey;
+
+            // Determine whether the tab state is worth saving.
+            let shouldSave = this._shouldSaveTabState(tabData.state);
+            let index = closedTabs.indexOf(tabData);
+
+            if (shouldSave && index == -1) {
+              // If the tab state is worth saving and we didn't push it onto
+              // the list of closed tabs when it was closed (because we deemed
+              // the state not worth saving) then add it to the window's list
+              // of closed tabs now.
+              this.saveClosedTabData(closedTabs, tabData);
+            } else if (!shouldSave && index > -1) {
+              // Remove from the list of closed tabs. The update messages sent
+              // after the tab was closed changed enough state so that we no
+              // longer consider its data interesting enough to keep around.
+              this.removeClosedTabData(closedTabs, index);
+            }
+          }
+        }
+        break;
+      case "SessionStore:restoreHistoryComplete":
+        // Notify the tabbrowser that the tab chrome has been restored.
+        let tabData = TabState.collect(tab);
+
+        // wall-paper fix for bug 439675: make sure that the URL to be loaded
+        // is always visible in the address bar if no other value is present
+        let activePageData = tabData.entries[tabData.index - 1] || null;
+        let uri = activePageData ? activePageData.url || null : null;
+        // NB: we won't set initial URIs (about:home, about:newtab, etc.) here
+        // because their load will not normally trigger a location bar clearing
+        // when they finish loading (to avoid race conditions where we then
+        // clear user input instead), so we shouldn't set them here either.
+        // They also don't fall under the issues in bug 439675 where user input
+        // needs to be preserved if the load doesn't succeed.
+        // We also don't do this for remoteness updates, where it should not
+        // be necessary.
+        if (!browser.userTypedValue && uri && !data.isRemotenessUpdate &&
+            !win.gInitialPages.includes(uri)) {
+          browser.userTypedValue = uri;
+        }
+
+        // If the page has a title, set it.
+        if (activePageData) {
+          if (activePageData.title) {
+            tab.label = activePageData.title;
+            tab.crop = "end";
+          } else if (activePageData.url != "about:blank") {
+            tab.label = activePageData.url;
+            tab.crop = "center";
+          }
+        } else if (tab.hasAttribute("customizemode")) {
+          win.gCustomizeMode.setTab(tab);
+        }
+
+        // Restore the tab icon.
+        if ("image" in tabData) {
+          // Use the serialized contentPrincipal with the new icon load.
+          let loadingPrincipal = Utils.deserializePrincipal(tabData.iconLoadingPrincipal);
+          win.gBrowser.setIcon(tab, tabData.image, loadingPrincipal);
+          TabStateCache.update(browser, { image: null, iconLoadingPrincipal: null });
+        }
+
+        let event = win.document.createEvent("Events");
+        event.initEvent("SSTabRestoring", true, false);
+        tab.dispatchEvent(event);
+        break;
+      case "SessionStore:restoreTabContentStarted":
+        if (browser.__SS_restoreState == TAB_STATE_NEEDS_RESTORE) {
+          // If a load not initiated by sessionstore was started in a
+          // previously pending tab. Mark the tab as no longer pending.
+          this.markTabAsRestoring(tab);
+        } else if (!data.isRemotenessUpdate) {
+          // If the user was typing into the URL bar when we crashed, but hadn't hit
+          // enter yet, then we just need to write that value to the URL bar without
+          // loading anything. This must happen after the load, as the load will clear
+          // userTypedValue.
+          let tabData = TabState.collect(tab);
+          if (tabData.userTypedValue && !tabData.userTypedClear && !browser.userTypedValue) {
+            browser.userTypedValue = tabData.userTypedValue;
+            win.URLBarSetURI();
+          }
+
+          // Remove state we don't need any longer.
+          TabStateCache.update(browser, {
+            userTypedValue: null, userTypedClear: null
+          });
+        }
+        break;
+      case "SessionStore:restoreTabContentComplete":
+        // This callback is used exclusively by tests that want to
+        // monitor the progress of network loads.
+        if (gDebuggingEnabled) {
+          Services.obs.notifyObservers(browser, NOTIFY_TAB_RESTORED, null);
+        }
+
+        SessionStoreInternal._resetLocalTabRestoringState(tab);
+        SessionStoreInternal.restoreNextTab();
+
+        this._sendTabRestoredNotification(tab, data.isRemotenessUpdate);
+        break;
+      case "SessionStore:crashedTabRevived":
+        // The browser was revived by navigating to a different page
+        // manually, so we remove it from the ignored browser set.
+        this._crashedBrowsers.delete(browser.permanentKey);
+        break;
+      case "SessionStore:error":
+        this.reportInternalError(data);
+        TabStateFlusher.resolveAll(browser, false, "Received error from the content process");
+        break;
+      default:
+        throw new Error(`received unknown message '${aMessage.name}'`);
+        break;
+    }
+  },
+
+  /**
+   * Record telemetry measurements stored in an object.
+   * @param telemetry
+   *        {histogramID: value, ...} An object mapping histogramIDs to the
+   *        value to be recorded for that ID,
+   */
+  recordTelemetry: function (telemetry) {
+    for (let histogramId in telemetry){
+      Telemetry.getHistogramById(histogramId).add(telemetry[histogramId]);
+    }
+  },
+
+  /* ........ Window Event Handlers .............. */
+
+  /**
+   * Implement nsIDOMEventListener for handling various window and tab events
+   */
+  handleEvent: function ssi_handleEvent(aEvent) {
+    let win = aEvent.currentTarget.ownerGlobal;
+    let target = aEvent.originalTarget;
+    switch (aEvent.type) {
+      case "TabOpen":
+        this.onTabAdd(win);
+        break;
+      case "TabBrowserInserted":
+        this.onTabBrowserInserted(win, target);
+        break;
+      case "TabClose":
+        // `adoptedBy` will be set if the tab was closed because it is being
+        // moved to a new window.
+        if (!aEvent.detail.adoptedBy)
+          this.onTabClose(win, target);
+        this.onTabRemove(win, target);
+        break;
+      case "TabSelect":
+        this.onTabSelect(win);
+        break;
+      case "TabShow":
+        this.onTabShow(win, target);
+        break;
+      case "TabHide":
+        this.onTabHide(win, target);
+        break;
+      case "TabPinned":
+      case "TabUnpinned":
+      case "SwapDocShells":
+        this.saveStateDelayed(win);
+        break;
+      case "oop-browser-crashed":
+        this.onBrowserCrashed(target);
+        break;
+      case "XULFrameLoaderCreated":
+        if (target.namespaceURI == NS_XUL &&
+            target.localName == "browser" &&
+            target.frameLoader &&
+            target.permanentKey) {
+          this._lastKnownFrameLoader.set(target.permanentKey, target.frameLoader);
+          this.resetEpoch(target);
+        }
+        break;
+      default:
+        throw new Error(`unhandled event ${aEvent.type}?`);
+    }
+    this._clearRestoringWindows();
+  },
+
+  /**
+   * Generate a unique window identifier
+   * @return string
+   *         A unique string to identify a window
+   */
+  _generateWindowID: function ssi_generateWindowID() {
+    return "window" + (this._nextWindowID++);
+  },
+
+  /**
+   * Registers and tracks a given window.
+   *
+   * @param aWindow
+   *        Window reference
+   */
+  onLoad(aWindow) {
+    // return if window has already been initialized
+    if (aWindow && aWindow.__SSi && this._windows[aWindow.__SSi])
+      return;
+
+    // ignore windows opened while shutting down
+    if (RunState.isQuitting)
+      return;
+
+    // Assign the window a unique identifier we can use to reference
+    // internal data about the window.
+    aWindow.__SSi = this._generateWindowID();
+
+    let mm = aWindow.getGroupMessageManager("browsers");
+    MESSAGES.forEach(msg => {
+      let listenWhenClosed = CLOSED_MESSAGES.has(msg);
+      mm.addMessageListener(msg, this, listenWhenClosed);
+    });
+
+    // Load the frame script after registering listeners.
+    mm.loadFrameScript("chrome://browser/content/content-sessionStore.js", true);
+
+    // and create its data object
+    this._windows[aWindow.__SSi] = { tabs: [], selected: 0, _closedTabs: [], busy: false };
+
+    if (PrivateBrowsingUtils.isWindowPrivate(aWindow))
+      this._windows[aWindow.__SSi].isPrivate = true;
+    if (!this._isWindowLoaded(aWindow))
+      this._windows[aWindow.__SSi]._restoring = true;
+    if (!aWindow.toolbar.visible)
+      this._windows[aWindow.__SSi].isPopup = true;
+
+    let tabbrowser = aWindow.gBrowser;
+
+    // add tab change listeners to all already existing tabs
+    for (let i = 0; i < tabbrowser.tabs.length; i++) {
+      this.onTabBrowserInserted(aWindow, tabbrowser.tabs[i]);
+    }
+    // notification of tab add/remove/selection/show/hide
+    TAB_EVENTS.forEach(function(aEvent) {
+      tabbrowser.tabContainer.addEventListener(aEvent, this, true);
+    }, this);
+
+    // Keep track of a browser's latest frameLoader.
+    aWindow.gBrowser.addEventListener("XULFrameLoaderCreated", this);
+  },
+
+  /**
+   * Initializes a given window.
+   *
+   * Windows are registered as soon as they are created but we need to wait for
+   * the session file to load, and the initial window's delayed startup to
+   * finish before initializing a window, i.e. restoring data into it.
+   *
+   * @param aWindow
+   *        Window reference
+   * @param aInitialState
+   *        The initial state to be loaded after startup (optional)
+   */
+  initializeWindow(aWindow, aInitialState = null) {
+    let isPrivateWindow = PrivateBrowsingUtils.isWindowPrivate(aWindow);
+
+    // perform additional initialization when the first window is loading
+    if (RunState.isStopped) {
+      RunState.setRunning();
+
+      // restore a crashed session resp. resume the last session if requested
+      if (aInitialState) {
+        // Don't write to disk right after startup. Set the last time we wrote
+        // to disk to NOW() to enforce a full interval before the next write.
+        SessionSaver.updateLastSaveTime();
+
+        if (isPrivateWindow) {
+          // We're starting with a single private window. Save the state we
+          // actually wanted to restore so that we can do it later in case
+          // the user opens another, non-private window.
+          this._deferredInitialState = gSessionStartup.state;
+
+          // Nothing to restore now, notify observers things are complete.
+          Services.obs.notifyObservers(null, NOTIFY_WINDOWS_RESTORED, "");
+        } else {
+          TelemetryTimestamps.add("sessionRestoreRestoring");
+          this._restoreCount = aInitialState.windows ? aInitialState.windows.length : 0;
+
+          // global data must be restored before restoreWindow is called so that
+          // it happens before observers are notified
+          this._globalState.setFromState(aInitialState);
+
+          let overwrite = this._isCmdLineEmpty(aWindow, aInitialState);
+          let options = {firstWindow: true, overwriteTabs: overwrite};
+          this.restoreWindows(aWindow, aInitialState, options);
+        }
+      }
+      else {
+        // Nothing to restore, notify observers things are complete.
+        Services.obs.notifyObservers(null, NOTIFY_WINDOWS_RESTORED, "");
+      }
+    }
+    // this window was opened by _openWindowWithState
+    else if (!this._isWindowLoaded(aWindow)) {
+      let state = this._statesToRestore[aWindow.__SS_restoreID];
+      let options = {overwriteTabs: true, isFollowUp: state.windows.length == 1};
+      this.restoreWindow(aWindow, state.windows[0], options);
+    }
+    // The user opened another, non-private window after starting up with
+    // a single private one. Let's restore the session we actually wanted to
+    // restore at startup.
+    else if (this._deferredInitialState && !isPrivateWindow &&
+             aWindow.toolbar.visible) {
+
+      // global data must be restored before restoreWindow is called so that
+      // it happens before observers are notified
+      this._globalState.setFromState(this._deferredInitialState);
+
+      this._restoreCount = this._deferredInitialState.windows ?
+        this._deferredInitialState.windows.length : 0;
+      this.restoreWindows(aWindow, this._deferredInitialState, {firstWindow: true});
+      this._deferredInitialState = null;
+    }
+    else if (this._restoreLastWindow && aWindow.toolbar.visible &&
+             this._closedWindows.length && !isPrivateWindow) {
+
+      // default to the most-recently closed window
+      // don't use popup windows
+      let closedWindowState = null;
+      let closedWindowIndex;
+      for (let i = 0; i < this._closedWindows.length; i++) {
+        // Take the first non-popup, point our object at it, and break out.
+        if (!this._closedWindows[i].isPopup) {
+          closedWindowState = this._closedWindows[i];
+          closedWindowIndex = i;
+          break;
+        }
+      }
+
+      if (closedWindowState) {
+        let newWindowState;
+        if (AppConstants.platform == "macosx" || !this._doResumeSession()) {
+          // We want to split the window up into pinned tabs and unpinned tabs.
+          // Pinned tabs should be restored. If there are any remaining tabs,
+          // they should be added back to _closedWindows.
+          // We'll cheat a little bit and reuse _prepDataForDeferredRestore
+          // even though it wasn't built exactly for this.
+          let [appTabsState, normalTabsState] =
+            this._prepDataForDeferredRestore({ windows: [closedWindowState] });
+
+          // These are our pinned tabs, which we should restore
+          if (appTabsState.windows.length) {
+            newWindowState = appTabsState.windows[0];
+            delete newWindowState.__lastSessionWindowID;
+          }
+
+          // In case there were no unpinned tabs, remove the window from _closedWindows
+          if (!normalTabsState.windows.length) {
+            this._closedWindows.splice(closedWindowIndex, 1);
+          }
+          // Or update _closedWindows with the modified state
+          else {
+            delete normalTabsState.windows[0].__lastSessionWindowID;
+            this._closedWindows[closedWindowIndex] = normalTabsState.windows[0];
+          }
+        }
+        else {
+          // If we're just restoring the window, make sure it gets removed from
+          // _closedWindows.
+          this._closedWindows.splice(closedWindowIndex, 1);
+          newWindowState = closedWindowState;
+          delete newWindowState.hidden;
+        }
+
+        if (newWindowState) {
+          // Ensure that the window state isn't hidden
+          this._restoreCount = 1;
+          let state = { windows: [newWindowState] };
+          let options = {overwriteTabs: this._isCmdLineEmpty(aWindow, state)};
+          this.restoreWindow(aWindow, newWindowState, options);
+        }
+      }
+      // we actually restored the session just now.
+      this._prefBranch.setBoolPref("sessionstore.resume_session_once", false);
+    }
+    if (this._restoreLastWindow && aWindow.toolbar.visible) {
+      // always reset (if not a popup window)
+      // we don't want to restore a window directly after, for example,
+      // undoCloseWindow was executed.
+      this._restoreLastWindow = false;
+    }
+  },
+
+  /**
+   * Called right before a new browser window is shown.
+   * @param aWindow
+   *        Window reference
+   */
+  onBeforeBrowserWindowShown: function (aWindow) {
+    // Register the window.
+    this.onLoad(aWindow);
+
+    // Just call initializeWindow() directly if we're initialized already.
+    if (this._sessionInitialized) {
+      this.initializeWindow(aWindow);
+      return;
+    }
+
+    // The very first window that is opened creates a promise that is then
+    // re-used by all subsequent windows. The promise will be used to tell
+    // when we're ready for initialization.
+    if (!this._promiseReadyForInitialization) {
+      // Wait for the given window's delayed startup to be finished.
+      let promise = new Promise(resolve => {
+        Services.obs.addObserver(function obs(subject, topic) {
+          if (aWindow == subject) {
+            Services.obs.removeObserver(obs, topic);
+            resolve();
+          }
+        }, "browser-delayed-startup-finished", false);
+      });
+
+      // We are ready for initialization as soon as the session file has been
+      // read from disk and the initial window's delayed startup has finished.
+      this._promiseReadyForInitialization =
+        Promise.all([promise, gSessionStartup.onceInitialized]);
+    }
+
+    // We can't call this.onLoad since initialization
+    // hasn't completed, so we'll wait until it is done.
+    // Even if additional windows are opened and wait
+    // for initialization as well, the first opened
+    // window should execute first, and this.onLoad
+    // will be called with the initialState.
+    this._promiseReadyForInitialization.then(() => {
+      if (aWindow.closed) {
+        return;
+      }
+
+      if (this._sessionInitialized) {
+        this.initializeWindow(aWindow);
+      } else {
+        let initialState = this.initSession();
+        this._sessionInitialized = true;
+
+        if (initialState) {
+          Services.obs.notifyObservers(null, NOTIFY_RESTORING_ON_STARTUP, "");
+        }
+        TelemetryStopwatch.start("FX_SESSION_RESTORE_STARTUP_ONLOAD_INITIAL_WINDOW_MS");
+        this.initializeWindow(aWindow, initialState);
+        TelemetryStopwatch.finish("FX_SESSION_RESTORE_STARTUP_ONLOAD_INITIAL_WINDOW_MS");
+
+        // Let everyone know we're done.
+        this._deferredInitialized.resolve();
+      }
+    }, console.error);
+  },
+
+  /**
+   * On window close...
+   * - remove event listeners from tabs
+   * - save all window data
+   * @param aWindow
+   *        Window reference
+   */
+  onClose: function ssi_onClose(aWindow) {
+    // this window was about to be restored - conserve its original data, if any
+    let isFullyLoaded = this._isWindowLoaded(aWindow);
+    if (!isFullyLoaded) {
+      if (!aWindow.__SSi) {
+        aWindow.__SSi = this._generateWindowID();
+      }
+
+      this._windows[aWindow.__SSi] = this._statesToRestore[aWindow.__SS_restoreID];
+      delete this._statesToRestore[aWindow.__SS_restoreID];
+      delete aWindow.__SS_restoreID;
+    }
+
+    // ignore windows not tracked by SessionStore
+    if (!aWindow.__SSi || !this._windows[aWindow.__SSi]) {
+      return;
+    }
+
+    // notify that the session store will stop tracking this window so that
+    // extensions can store any data about this window in session store before
+    // that's not possible anymore
+    let event = aWindow.document.createEvent("Events");
+    event.initEvent("SSWindowClosing", true, false);
+    aWindow.dispatchEvent(event);
+
+    if (this.windowToFocus && this.windowToFocus == aWindow) {
+      delete this.windowToFocus;
+    }
+
+    var tabbrowser = aWindow.gBrowser;
+
+    let browsers = Array.from(tabbrowser.browsers);
+
+    TAB_EVENTS.forEach(function(aEvent) {
+      tabbrowser.tabContainer.removeEventListener(aEvent, this, true);
+    }, this);
+
+    aWindow.gBrowser.removeEventListener("XULFrameLoaderCreated", this);
+
+    let winData = this._windows[aWindow.__SSi];
+
+    // Collect window data only when *not* closed during shutdown.
+    if (RunState.isRunning) {
+      // Grab the most recent window data. The tab data will be updated
+      // once we finish flushing all of the messages from the tabs.
+      let tabMap = this._collectWindowData(aWindow);
+
+      for (let [tab, tabData] of tabMap) {
+        let permanentKey = tab.linkedBrowser.permanentKey;
+        this._closedWindowTabs.set(permanentKey, tabData);
+      }
+
+      if (isFullyLoaded) {
+        winData.title = tabbrowser.selectedBrowser.contentTitle || tabbrowser.selectedTab.label;
+        winData.title = this._replaceLoadingTitle(winData.title, tabbrowser,
+                                                  tabbrowser.selectedTab);
+        SessionCookies.update([winData]);
+      }
+
+      if (AppConstants.platform != "macosx") {
+        // Until we decide otherwise elsewhere, this window is part of a series
+        // of closing windows to quit.
+        winData._shouldRestore = true;
+      }
+
+      // Store the window's close date to figure out when each individual tab
+      // was closed. This timestamp should allow re-arranging data based on how
+      // recently something was closed.
+      winData.closedAt = Date.now();
+
+      // we don't want to save the busy state
+      delete winData.busy;
+
+      // When closing windows one after the other until Firefox quits, we
+      // will move those closed in series back to the "open windows" bucket
+      // before writing to disk. If however there is only a single window
+      // with tabs we deem not worth saving then we might end up with a
+      // random closed or even a pop-up window re-opened. To prevent that
+      // we explicitly allow saving an "empty" window state.
+      let isLastWindow =
+        Object.keys(this._windows).length == 1 &&
+        !this._closedWindows.some(win => win._shouldRestore || false);
+
+      // clear this window from the list, since it has definitely been closed.
+      delete this._windows[aWindow.__SSi];
+
+      // This window has the potential to be saved in the _closedWindows
+      // array (maybeSaveClosedWindows gets the final call on that).
+      this._saveableClosedWindowData.add(winData);
+
+      // Now we have to figure out if this window is worth saving in the _closedWindows
+      // Object.
+      //
+      // We're about to flush the tabs from this window, but it's possible that we
+      // might never hear back from the content process(es) in time before the user
+      // chooses to restore the closed window. So we do the following:
+      //
+      // 1) Use the tab state cache to determine synchronously if the window is
+      //    worth stashing in _closedWindows.
+      // 2) Flush the window.
+      // 3) When the flush is complete, revisit our decision to store the window
+      //    in _closedWindows, and add/remove as necessary.
+      if (!winData.isPrivate) {
+        // Remove any open private tabs the window may contain.
+        PrivacyFilter.filterPrivateTabs(winData);
+        this.maybeSaveClosedWindow(winData, isLastWindow);
+      }
+
+      TabStateFlusher.flushWindow(aWindow).then(() => {
+        // At this point, aWindow is closed! You should probably not try to
+        // access any DOM elements from aWindow within this callback unless
+        // you're holding on to them in the closure.
+
+        for (let browser of browsers) {
+          if (this._closedWindowTabs.has(browser.permanentKey)) {
+            let tabData = this._closedWindowTabs.get(browser.permanentKey);
+            TabState.copyFromCache(browser, tabData);
+            this._closedWindowTabs.delete(browser.permanentKey);
+          }
+        }
+
+        // Save non-private windows if they have at
+        // least one saveable tab or are the last window.
+        if (!winData.isPrivate) {
+          // It's possible that a tab switched its privacy state at some point
+          // before our flush, so we need to filter again.
+          PrivacyFilter.filterPrivateTabs(winData);
+          this.maybeSaveClosedWindow(winData, isLastWindow);
+        }
+
+        // Update the tabs data now that we've got the most
+        // recent information.
+        this.cleanUpWindow(aWindow, winData, browsers);
+
+        // save the state without this window to disk
+        this.saveStateDelayed();
+      });
+    } else {
+      this.cleanUpWindow(aWindow, winData, browsers);
+    }
+
+    for (let i = 0; i < tabbrowser.tabs.length; i++) {
+      this.onTabRemove(aWindow, tabbrowser.tabs[i], true);
+    }
+  },
+
+  /**
+   * Clean up the message listeners on a window that has finally
+   * gone away. Call this once you're sure you don't want to hear
+   * from any of this windows tabs from here forward.
+   *
+   * @param aWindow
+   *        The browser window we're cleaning up.
+   * @param winData
+   *        The data for the window that we should hold in the
+   *        DyingWindowCache in case anybody is still holding a
+   *        reference to it.
+   */
+  cleanUpWindow(aWindow, winData, browsers) {
+    // Any leftover TabStateFlusher Promises need to be resolved now,
+    // since we're about to remove the message listeners.
+    for (let browser of browsers) {
+      TabStateFlusher.resolveAll(browser);
+    }
+
+    // Cache the window state until it is completely gone.
+    DyingWindowCache.set(aWindow, winData);
+
+    let mm = aWindow.getGroupMessageManager("browsers");
+    MESSAGES.forEach(msg => mm.removeMessageListener(msg, this));
+
+    this._saveableClosedWindowData.delete(winData);
+    delete aWindow.__SSi;
+  },
+
+  /**
+   * Decides whether or not a closed window should be put into the
+   * _closedWindows Object. This might be called multiple times per
+   * window, and will do the right thing of moving the window data
+   * in or out of _closedWindows if the winData indicates that our
+   * need for saving it has changed.
+   *
+   * @param winData
+   *        The data for the closed window that we might save.
+   * @param isLastWindow
+   *        Whether or not the window being closed is the last
+   *        browser window. Callers of this function should pass
+   *        in the value of SessionStoreInternal.atLastWindow for
+   *        this argument, and pass in the same value if they happen
+   *        to call this method again asynchronously (for example, after
+   *        a window flush).
+   */
+  maybeSaveClosedWindow(winData, isLastWindow) {
+    // Make sure SessionStore is still running, and make sure that we
+    // haven't chosen to forget this window.
+    if (RunState.isRunning && this._saveableClosedWindowData.has(winData)) {
+      // Determine whether the window has any tabs worth saving.
+      let hasSaveableTabs = winData.tabs.some(this._shouldSaveTabState);
+
+      // Note that we might already have this window stored in
+      // _closedWindows from a previous call to this function.
+      let winIndex = this._closedWindows.indexOf(winData);
+      let alreadyStored = (winIndex != -1);
+      let shouldStore = (hasSaveableTabs || isLastWindow);
+
+      if (shouldStore && !alreadyStored) {
+        let index = this._closedWindows.findIndex(win => {
+          return win.closedAt < winData.closedAt;
+        });
+
+        // If we found no tab closed before our
+        // tab then just append it to the list.
+        if (index == -1) {
+          index = this._closedWindows.length;
+        }
+
+        // About to save the closed window, add a unique ID.
+        winData.closedId = this._nextClosedId++;
+
+        // Insert tabData at the right position.
+        this._closedWindows.splice(index, 0, winData);
+        this._capClosedWindows();
+      } else if (!shouldStore && alreadyStored) {
+        this._closedWindows.splice(winIndex, 1);
+      }
+    }
+  },
+
+  /**
+   * On quit application granted
+   */
+  onQuitApplicationGranted: function ssi_onQuitApplicationGranted(syncShutdown=false) {
+    // Collect an initial snapshot of window data before we do the flush
+    this._forEachBrowserWindow((win) => {
+      this._collectWindowData(win);
+    });
+
+    // Now add an AsyncShutdown blocker that'll spin the event loop
+    // until the windows have all been flushed.
+
+    // This progress object will track the state of async window flushing
+    // and will help us debug things that go wrong with our AsyncShutdown
+    // blocker.
+    let progress = { total: -1, current: -1 };
+
+    // We're going down! Switch state so that we treat closing windows and
+    // tabs correctly.
+    RunState.setQuitting();
+
+    if (!syncShutdown) {
+      // We've got some time to shut down, so let's do this properly.
+      // To prevent blocker from breaking the 60 sec limit(which will cause a
+      // crash) of async shutdown during flushing all windows, we resolve the
+      // promise passed to blocker once:
+      // 1. the flushing exceed 50 sec, or
+      // 2. 'oop-frameloader-crashed' or 'ipc:content-shutdown' is observed.
+      // Thus, Firefox still can open the last session on next startup.
+      AsyncShutdown.quitApplicationGranted.addBlocker(
+        "SessionStore: flushing all windows",
+        () => {
+          var promises = [];
+          promises.push(this.flushAllWindowsAsync(progress));
+          promises.push(this.looseTimer(50000));
+
+          var promiseOFC = new Promise(resolve => {
+            Services.obs.addObserver(function obs(subject, topic) {
+              Services.obs.removeObserver(obs, topic);
+              resolve();
+            }, "oop-frameloader-crashed", false);
+          });
+          promises.push(promiseOFC);
+
+          var promiseICS = new Promise(resolve => {
+            Services.obs.addObserver(function obs(subject, topic) {
+              Services.obs.removeObserver(obs, topic);
+              resolve();
+            }, "ipc:content-shutdown", false);
+          });
+          promises.push(promiseICS);
+
+          return Promise.race(promises);
+        },
+        () => progress);
+    } else {
+      // We have to shut down NOW, which means we only get to save whatever
+      // we already had cached.
+    }
+  },
+
+  /**
+   * An async Task that iterates all open browser windows and flushes
+   * any outstanding messages from their tabs. This will also close
+   * all of the currently open windows while we wait for the flushes
+   * to complete.
+   *
+   * @param progress (Object)
+   *        Optional progress object that will be updated as async
+   *        window flushing progresses. flushAllWindowsSync will
+   *        write to the following properties:
+   *
+   *        total (int):
+   *          The total number of windows to be flushed.
+   *        current (int):
+   *          The current window that we're waiting for a flush on.
+   *
+   * @return Promise
+   */
+  flushAllWindowsAsync: Task.async(function*(progress={}) {
+    let windowPromises = new Map();
+    // We collect flush promises and close each window immediately so that
+    // the user can't start changing any window state while we're waiting
+    // for the flushes to finish.
+    this._forEachBrowserWindow((win) => {
+      windowPromises.set(win, TabStateFlusher.flushWindow(win));
+
+      // We have to wait for these messages to come up from
+      // each window and each browser. In the meantime, hide
+      // the windows to improve perceived shutdown speed.
+      let baseWin = win.QueryInterface(Ci.nsIInterfaceRequestor)
+                       .getInterface(Ci.nsIDocShell)
+                       .QueryInterface(Ci.nsIDocShellTreeItem)
+                       .treeOwner
+                       .QueryInterface(Ci.nsIBaseWindow);
+      baseWin.visibility = false;
+    });
+
+    progress.total = windowPromises.size;
+    progress.current = 0;
+
+    // We'll iterate through the Promise array, yielding each one, so as to
+    // provide useful progress information to AsyncShutdown.
+    for (let [win, promise] of windowPromises) {
+      yield promise;
+      this._collectWindowData(win);
+      progress.current++;
+    };
+
+    // We must cache this because _getMostRecentBrowserWindow will always
+    // return null by the time quit-application occurs.
+    var activeWindow = this._getMostRecentBrowserWindow();
+    if (activeWindow)
+      this.activeWindowSSiCache = activeWindow.__SSi || "";
+    DirtyWindows.clear();
+  }),
+
+  /**
+   * On last browser window close
+   */
+  onLastWindowCloseGranted: function ssi_onLastWindowCloseGranted() {
+    // last browser window is quitting.
+    // remember to restore the last window when another browser window is opened
+    // do not account for pref(resume_session_once) at this point, as it might be
+    // set by another observer getting this notice after us
+    this._restoreLastWindow = true;
+  },
+
+  /**
+   * On quitting application
+   * @param aData
+   *        String type of quitting
+   */
+  onQuitApplication: function ssi_onQuitApplication(aData) {
+    if (aData == "restart") {
+      this._prefBranch.setBoolPref("sessionstore.resume_session_once", true);
+      // The browser:purge-session-history notification fires after the
+      // quit-application notification so unregister the
+      // browser:purge-session-history notification to prevent clearing
+      // session data on disk on a restart.  It is also unnecessary to
+      // perform any other sanitization processing on a restart as the
+      // browser is about to exit anyway.
+      Services.obs.removeObserver(this, "browser:purge-session-history");
+    }
+
+    if (aData != "restart") {
+      // Throw away the previous session on shutdown
+      LastSession.clear();
+    }
+
+    this._uninit();
+  },
+
+  /**
+   * On purge of session history
+   */
+  onPurgeSessionHistory: function ssi_onPurgeSessionHistory() {
+    SessionFile.wipe();
+    // If the browser is shutting down, simply return after clearing the
+    // session data on disk as this notification fires after the
+    // quit-application notification so the browser is about to exit.
+    if (RunState.isQuitting)
+      return;
+    LastSession.clear();
+
+    let openWindows = {};
+    // Collect open windows.
+    this._forEachBrowserWindow(({__SSi: id}) => openWindows[id] = true);
+
+    // also clear all data about closed tabs and windows
+    for (let ix in this._windows) {
+      if (ix in openWindows) {
+        this._windows[ix]._closedTabs = [];
+      } else {
+        delete this._windows[ix];
+      }
+    }
+    // also clear all data about closed windows
+    this._closedWindows = [];
+    // give the tabbrowsers a chance to clear their histories first
+    var win = this._getMostRecentBrowserWindow();
+    if (win) {
+      win.setTimeout(() => SessionSaver.run(), 0);
+    } else if (RunState.isRunning) {
+      SessionSaver.run();
+    }
+
+    this._clearRestoringWindows();
+    this._saveableClosedWindowData = new WeakSet();
+  },
+
+  /**
+   * On purge of domain data
+   * @param aData
+   *        String domain data
+   */
+  onPurgeDomainData: function ssi_onPurgeDomainData(aData) {
+    // does a session history entry contain a url for the given domain?
+    function containsDomain(aEntry) {
+      if (Utils.hasRootDomain(aEntry.url, aData)) {
+        return true;
+      }
+      return aEntry.children && aEntry.children.some(containsDomain, this);
+    }
+    // remove all closed tabs containing a reference to the given domain
+    for (let ix in this._windows) {
+      let closedTabs = this._windows[ix]._closedTabs;
+      for (let i = closedTabs.length - 1; i >= 0; i--) {
+        if (closedTabs[i].state.entries.some(containsDomain, this))
+          closedTabs.splice(i, 1);
+      }
+    }
+    // remove all open & closed tabs containing a reference to the given
+    // domain in closed windows
+    for (let ix = this._closedWindows.length - 1; ix >= 0; ix--) {
+      let closedTabs = this._closedWindows[ix]._closedTabs;
+      let openTabs = this._closedWindows[ix].tabs;
+      let openTabCount = openTabs.length;
+      for (let i = closedTabs.length - 1; i >= 0; i--)
+        if (closedTabs[i].state.entries.some(containsDomain, this))
+          closedTabs.splice(i, 1);
+      for (let j = openTabs.length - 1; j >= 0; j--) {
+        if (openTabs[j].entries.some(containsDomain, this)) {
+          openTabs.splice(j, 1);
+          if (this._closedWindows[ix].selected > j)
+            this._closedWindows[ix].selected--;
+        }
+      }
+      if (openTabs.length == 0) {
+        this._closedWindows.splice(ix, 1);
+      }
+      else if (openTabs.length != openTabCount) {
+        // Adjust the window's title if we removed an open tab
+        let selectedTab = openTabs[this._closedWindows[ix].selected - 1];
+        // some duplication from restoreHistory - make sure we get the correct title
+        let activeIndex = (selectedTab.index || selectedTab.entries.length) - 1;
+        if (activeIndex >= selectedTab.entries.length)
+          activeIndex = selectedTab.entries.length - 1;
+        this._closedWindows[ix].title = selectedTab.entries[activeIndex].title;
+      }
+    }
+
+    if (RunState.isRunning) {
+      SessionSaver.run();
+    }
+
+    this._clearRestoringWindows();
+  },
+
+  /**
+   * On preference change
+   * @param aData
+   *        String preference changed
+   */
+  onPrefChange: function ssi_onPrefChange(aData) {
+    switch (aData) {
+      // if the user decreases the max number of closed tabs they want
+      // preserved update our internal states to match that max
+      case "sessionstore.max_tabs_undo":
+        this._max_tabs_undo = this._prefBranch.getIntPref("sessionstore.max_tabs_undo");
+        for (let ix in this._windows) {
+          this._windows[ix]._closedTabs.splice(this._max_tabs_undo, this._windows[ix]._closedTabs.length);
+        }
+        break;
+      case "sessionstore.max_windows_undo":
+        this._max_windows_undo = this._prefBranch.getIntPref("sessionstore.max_windows_undo");
+        this._capClosedWindows();
+        break;
+    }
+  },
+
+  /**
+   * save state when new tab is added
+   * @param aWindow
+   *        Window reference
+   */
+  onTabAdd: function ssi_onTabAdd(aWindow) {
+    this.saveStateDelayed(aWindow);
+  },
+
+  /**
+   * set up listeners for a new tab
+   * @param aWindow
+   *        Window reference
+   * @param aTab
+   *        Tab reference
+   */
+  onTabBrowserInserted: function ssi_onTabBrowserInserted(aWindow, aTab) {
+    let browser = aTab.linkedBrowser;
+    browser.addEventListener("SwapDocShells", this);
+    browser.addEventListener("oop-browser-crashed", this);
+
+    if (browser.frameLoader) {
+      this._lastKnownFrameLoader.set(browser.permanentKey, browser.frameLoader);
+    }
+  },
+
+  /**
+   * remove listeners for a tab
+   * @param aWindow
+   *        Window reference
+   * @param aTab
+   *        Tab reference
+   * @param aNoNotification
+   *        bool Do not save state if we're updating an existing tab
+   */
+  onTabRemove: function ssi_onTabRemove(aWindow, aTab, aNoNotification) {
+    let browser = aTab.linkedBrowser;
+    browser.removeEventListener("SwapDocShells", this);
+    browser.removeEventListener("oop-browser-crashed", this);
+
+    // If this tab was in the middle of restoring or still needs to be restored,
+    // we need to reset that state. If the tab was restoring, we will attempt to
+    // restore the next tab.
+    let previousState = browser.__SS_restoreState;
+    if (previousState) {
+      this._resetTabRestoringState(aTab);
+      if (previousState == TAB_STATE_RESTORING)
+        this.restoreNextTab();
+    }
+
+    if (!aNoNotification) {
+      this.saveStateDelayed(aWindow);
+    }
+  },
+
+  /**
+   * When a tab closes, collect its properties
+   * @param aWindow
+   *        Window reference
+   * @param aTab
+   *        Tab reference
+   */
+  onTabClose: function ssi_onTabClose(aWindow, aTab) {
+    // notify the tabbrowser that the tab state will be retrieved for the last time
+    // (so that extension authors can easily set data on soon-to-be-closed tabs)
+    var event = aWindow.document.createEvent("Events");
+    event.initEvent("SSTabClosing", true, false);
+    aTab.dispatchEvent(event);
+
+    // don't update our internal state if we don't have to
+    if (this._max_tabs_undo == 0) {
+      return;
+    }
+
+    // Get the latest data for this tab (generally, from the cache)
+    let tabState = TabState.collect(aTab);
+
+    // Don't save private tabs
+    let isPrivateWindow = PrivateBrowsingUtils.isWindowPrivate(aWindow);
+    if (!isPrivateWindow && tabState.isPrivate) {
+      return;
+    }
+
+    // Store closed-tab data for undo.
+    let tabbrowser = aWindow.gBrowser;
+    let tabTitle = this._replaceLoadingTitle(aTab.label, tabbrowser, aTab);
+    let {permanentKey} = aTab.linkedBrowser;
+
+    let tabData = {
+      permanentKey,
+      state: tabState,
+      title: tabTitle,
+      image: tabbrowser.getIcon(aTab),
+      iconLoadingPrincipal: Utils.serializePrincipal(aTab.linkedBrowser.contentPrincipal),
+      pos: aTab._tPos,
+      closedAt: Date.now()
+    };
+
+    let closedTabs = this._windows[aWindow.__SSi]._closedTabs;
+
+    // Determine whether the tab contains any information worth saving. Note
+    // that there might be pending state changes queued in the child that
+    // didn't reach the parent yet. If a tab is emptied before closing then we
+    // might still remove it from the list of closed tabs later.
+    if (this._shouldSaveTabState(tabState)) {
+      // Save the tab state, for now. We might push a valid tab out
+      // of the list but those cases should be extremely rare and
+      // do probably never occur when using the browser normally.
+      // (Tests or add-ons might do weird things though.)
+      this.saveClosedTabData(closedTabs, tabData);
+    }
+
+    // Remember the closed tab to properly handle any last updates included in
+    // the final "update" message sent by the frame script's unload handler.
+    this._closedTabs.set(permanentKey, {closedTabs, tabData});
+  },
+
+  /**
+   * Insert a given |tabData| object into the list of |closedTabs|. We will
+   * determine the right insertion point based on the .closedAt properties of
+   * all tabs already in the list. The list will be truncated to contain a
+   * maximum of |this._max_tabs_undo| entries.
+   *
+   * @param closedTabs (array)
+   *        The list of closed tabs for a window.
+   * @param tabData (object)
+   *        The tabData to be inserted.
+   */
+  saveClosedTabData(closedTabs, tabData) {
+    // Find the index of the first tab in the list
+    // of closed tabs that was closed before our tab.
+    let index = closedTabs.findIndex(tab => {
+      return tab.closedAt < tabData.closedAt;
+    });
+
+    // If we found no tab closed before our
+    // tab then just append it to the list.
+    if (index == -1) {
+      index = closedTabs.length;
+    }
+
+    // About to save the closed tab, add a unique ID.
+    tabData.closedId = this._nextClosedId++;
+
+    // Insert tabData at the right position.
+    closedTabs.splice(index, 0, tabData);
+
+    // Truncate the list of closed tabs, if needed.
+    if (closedTabs.length > this._max_tabs_undo) {
+      closedTabs.splice(this._max_tabs_undo, closedTabs.length);
+    }
+  },
+
+  /**
+   * Remove the closed tab data at |index| from the list of |closedTabs|. If
+   * the tab's final message is still pending we will simply discard it when
+   * it arrives so that the tab doesn't reappear in the list.
+   *
+   * @param closedTabs (array)
+   *        The list of closed tabs for a window.
+   * @param index (uint)
+   *        The index of the tab to remove.
+   */
+  removeClosedTabData(closedTabs, index) {
+    // Remove the given index from the list.
+    let [closedTab] = closedTabs.splice(index, 1);
+
+    // If the closed tab's state still has a .permanentKey property then we
+    // haven't seen its final update message yet. Remove it from the map of
+    // closed tabs so that we will simply discard its last messages and will
+    // not add it back to the list of closed tabs again.
+    if (closedTab.permanentKey) {
+      this._closedTabs.delete(closedTab.permanentKey);
+      this._closedWindowTabs.delete(closedTab.permanentKey);
+      delete closedTab.permanentKey;
+    }
+
+    return closedTab;
+  },
+
+  /**
+   * When a tab is selected, save session data
+   * @param aWindow
+   *        Window reference
+   */
+  onTabSelect: function ssi_onTabSelect(aWindow) {
+    if (RunState.isRunning) {
+      this._windows[aWindow.__SSi].selected = aWindow.gBrowser.tabContainer.selectedIndex;
+
+      let tab = aWindow.gBrowser.selectedTab;
+      let browser = tab.linkedBrowser;
+
+      if (browser.__SS_restoreState &&
+          browser.__SS_restoreState == TAB_STATE_NEEDS_RESTORE) {
+        // If __SS_restoreState is still on the browser and it is
+        // TAB_STATE_NEEDS_RESTORE, then then we haven't restored
+        // this tab yet.
+        //
+        // It's possible that this tab was recently revived, and that
+        // we've deferred showing the tab crashed page for it (if the
+        // tab crashed in the background). If so, we need to re-enter
+        // the crashed state, since we'll be showing the tab crashed
+        // page.
+        if (TabCrashHandler.willShowCrashedTab(browser)) {
+          this.enterCrashedState(browser);
+        } else {
+          this.restoreTabContent(tab);
+        }
+      }
+    }
+  },
+
+  onTabShow: function ssi_onTabShow(aWindow, aTab) {
+    // If the tab hasn't been restored yet, move it into the right bucket
+    if (aTab.linkedBrowser.__SS_restoreState &&
+        aTab.linkedBrowser.__SS_restoreState == TAB_STATE_NEEDS_RESTORE) {
+      TabRestoreQueue.hiddenToVisible(aTab);
+
+      // let's kick off tab restoration again to ensure this tab gets restored
+      // with "restore_hidden_tabs" == false (now that it has become visible)
+      this.restoreNextTab();
+    }
+
+    // Default delay of 2 seconds gives enough time to catch multiple TabShow
+    // events. This used to be due to changing groups in 'tab groups'. We
+    // might be able to get rid of this now?
+    this.saveStateDelayed(aWindow);
+  },
+
+  onTabHide: function ssi_onTabHide(aWindow, aTab) {
+    // If the tab hasn't been restored yet, move it into the right bucket
+    if (aTab.linkedBrowser.__SS_restoreState &&
+        aTab.linkedBrowser.__SS_restoreState == TAB_STATE_NEEDS_RESTORE) {
+      TabRestoreQueue.visibleToHidden(aTab);
+    }
+
+    // Default delay of 2 seconds gives enough time to catch multiple TabHide
+    // events. This used to be due to changing groups in 'tab groups'. We
+    // might be able to get rid of this now?
+    this.saveStateDelayed(aWindow);
+  },
+
+  /**
+   * Handler for the event that is fired when a <xul:browser> crashes.
+   *
+   * @param aWindow
+   *        The window that the crashed browser belongs to.
+   * @param aBrowser
+   *        The <xul:browser> that is now in the crashed state.
+   */
+  onBrowserCrashed: function(aBrowser) {
+    NS_ASSERT(aBrowser.isRemoteBrowser,
+              "Only remote browsers should be able to crash");
+
+    this.enterCrashedState(aBrowser);
+    // The browser crashed so we might never receive flush responses.
+    // Resolve all pending flush requests for the crashed browser.
+    TabStateFlusher.resolveAll(aBrowser);
+  },
+
+  /**
+   * Called when a browser is showing or is about to show the tab
+   * crashed page. This method causes SessionStore to ignore the
+   * tab until it's restored.
+   *
+   * @param browser
+   *        The <xul:browser> that is about to show the crashed page.
+   */
+  enterCrashedState(browser) {
+    this._crashedBrowsers.add(browser.permanentKey);
+
+    let win = browser.ownerGlobal;
+
+    // If we hadn't yet restored, or were still in the midst of
+    // restoring this browser at the time of the crash, we need
+    // to reset its state so that we can try to restore it again
+    // when the user revives the tab from the crash.
+    if (browser.__SS_restoreState) {
+      let tab = win.gBrowser.getTabForBrowser(browser);
+      this._resetLocalTabRestoringState(tab);
+    }
+  },
+
+  // Clean up data that has been closed a long time ago.
+  // Do not reschedule a save. This will wait for the next regular
+  // save.
+  onIdleDaily: function() {
+    // Remove old closed windows
+    this._cleanupOldData([this._closedWindows]);
+
+    // Remove closed tabs of closed windows
+    this._cleanupOldData(this._closedWindows.map((winData) => winData._closedTabs));
+
+    // Remove closed tabs of open windows
+    this._cleanupOldData(Object.keys(this._windows).map((key) => this._windows[key]._closedTabs));
+  },
+
+  // Remove "old" data from an array
+  _cleanupOldData: function(targets) {
+    const TIME_TO_LIVE = this._prefBranch.getIntPref("sessionstore.cleanup.forget_closed_after");
+    const now = Date.now();
+
+    for (let array of targets) {
+      for (let i = array.length - 1; i >= 0; --i)  {
+        let data = array[i];
+        // Make sure that we have a timestamp to tell us when the target
+        // has been closed. If we don't have a timestamp, default to a
+        // safe timestamp: just now.
+        data.closedAt = data.closedAt || now;
+        if (now - data.closedAt > TIME_TO_LIVE) {
+          array.splice(i, 1);
+        }
+      }
+    }
+  },
+
+  /* ........ nsISessionStore API .............. */
+
+  getBrowserState: function ssi_getBrowserState() {
+    let state = this.getCurrentState();
+
+    // Don't include the last session state in getBrowserState().
+    delete state.lastSessionState;
+
+    // Don't include any deferred initial state.
+    delete state.deferredInitialState;
+
+    return JSON.stringify(state);
+  },
+
+  setBrowserState: function ssi_setBrowserState(aState) {
+    this._handleClosedWindows();
+
+    try {
+      var state = JSON.parse(aState);
+    }
+    catch (ex) { /* invalid state object - don't restore anything */ }
+    if (!state) {
+      throw Components.Exception("Invalid state string: not JSON", Cr.NS_ERROR_INVALID_ARG);
+    }
+    if (!state.windows) {
+      throw Components.Exception("No windows", Cr.NS_ERROR_INVALID_ARG);
+    }
+
+    this._browserSetState = true;
+
+    // Make sure the priority queue is emptied out
+    this._resetRestoringState();
+
+    var window = this._getMostRecentBrowserWindow();
+    if (!window) {
+      this._restoreCount = 1;
+      this._openWindowWithState(state);
+      return;
+    }
+
+    // close all other browser windows
+    this._forEachBrowserWindow(function(aWindow) {
+      if (aWindow != window) {
+        aWindow.close();
+        this.onClose(aWindow);
+      }
+    });
+
+    // make sure closed window data isn't kept
+    this._closedWindows = [];
+
+    // determine how many windows are meant to be restored
+    this._restoreCount = state.windows ? state.windows.length : 0;
+
+    // global data must be restored before restoreWindow is called so that
+    // it happens before observers are notified
+    this._globalState.setFromState(state);
+
+    // restore to the given state
+    this.restoreWindows(window, state, {overwriteTabs: true});
+  },
+
+  getWindowState: function ssi_getWindowState(aWindow) {
+    if ("__SSi" in aWindow) {
+      return JSON.stringify(this._getWindowState(aWindow));
+    }
+
+    if (DyingWindowCache.has(aWindow)) {
+      let data = DyingWindowCache.get(aWindow);
+      return JSON.stringify({ windows: [data] });
+    }
+
+    throw Components.Exception("Window is not tracked", Cr.NS_ERROR_INVALID_ARG);
+  },
+
+  setWindowState: function ssi_setWindowState(aWindow, aState, aOverwrite) {
+    if (!aWindow.__SSi) {
+      throw Components.Exception("Window is not tracked", Cr.NS_ERROR_INVALID_ARG);
+    }
+
+    this.restoreWindows(aWindow, aState, {overwriteTabs: aOverwrite});
+  },
+
+  getTabState: function ssi_getTabState(aTab) {
+    if (!aTab.ownerGlobal.__SSi) {
+      throw Components.Exception("Default view is not tracked", Cr.NS_ERROR_INVALID_ARG);
+    }
+
+    let tabState = TabState.collect(aTab);
+
+    return JSON.stringify(tabState);
+  },
+
+  setTabState(aTab, aState) {
+    // Remove the tab state from the cache.
+    // Note that we cannot simply replace the contents of the cache
+    // as |aState| can be an incomplete state that will be completed
+    // by |restoreTabs|.
+    let tabState = JSON.parse(aState);
+    if (!tabState) {
+      throw Components.Exception("Invalid state string: not JSON", Cr.NS_ERROR_INVALID_ARG);
+    }
+    if (typeof tabState != "object") {
+      throw Components.Exception("Not an object", Cr.NS_ERROR_INVALID_ARG);
+    }
+    if (!("entries" in tabState)) {
+      throw Components.Exception("Invalid state object: no entries", Cr.NS_ERROR_INVALID_ARG);
+    }
+
+    let window = aTab.ownerGlobal;
+    if (!("__SSi" in window)) {
+      throw Components.Exception("Window is not tracked", Cr.NS_ERROR_INVALID_ARG);
+    }
+
+    if (aTab.linkedBrowser.__SS_restoreState) {
+      this._resetTabRestoringState(aTab);
+    }
+
+    this.restoreTab(aTab, tabState);
+  },
+
+  duplicateTab: function ssi_duplicateTab(aWindow, aTab, aDelta = 0, aRestoreImmediately = true) {
+    if (!aTab.ownerGlobal.__SSi) {
+      throw Components.Exception("Default view is not tracked", Cr.NS_ERROR_INVALID_ARG);
+    }
+    if (!aWindow.gBrowser) {
+      throw Components.Exception("Invalid window object: no gBrowser", Cr.NS_ERROR_INVALID_ARG);
+    }
+
+    // Create a new tab.
+    let userContextId = aTab.getAttribute("usercontextid");
+    let newTab = aTab == aWindow.gBrowser.selectedTab ?
+      aWindow.gBrowser.addTab(null, {relatedToCurrent: true, ownerTab: aTab, userContextId}) :
+      aWindow.gBrowser.addTab(null, {userContextId});
+
+    // Set tab title to "Connecting..." and start the throbber to pretend we're
+    // doing something while actually waiting for data from the frame script.
+    aWindow.gBrowser.setTabTitleLoading(newTab);
+    newTab.setAttribute("busy", "true");
+
+    // Collect state before flushing.
+    let tabState = TabState.clone(aTab);
+
+    // Flush to get the latest tab state to duplicate.
+    let browser = aTab.linkedBrowser;
+    TabStateFlusher.flush(browser).then(() => {
+      // The new tab might have been closed in the meantime.
+      if (newTab.closing || !newTab.linkedBrowser) {
+        return;
+      }
+
+      let window = newTab.ownerGlobal;
+
+      // The tab or its window might be gone.
+      if (!window || !window.__SSi) {
+        return;
+      }
+
+      // Update state with flushed data. We can't use TabState.clone() here as
+      // the tab to duplicate may have already been closed. In that case we
+      // only have access to the <xul:browser>.
+      let options = {includePrivateData: true};
+      TabState.copyFromCache(browser, tabState, options);
+
+      tabState.index += aDelta;
+      tabState.index = Math.max(1, Math.min(tabState.index, tabState.entries.length));
+      tabState.pinned = false;
+
+      // Restore the state into the new tab.
+      this.restoreTab(newTab, tabState, {
+        restoreImmediately: aRestoreImmediately
+      });
+    });
+
+    return newTab;
+  },
+
+  getClosedTabCount: function ssi_getClosedTabCount(aWindow) {
+    if ("__SSi" in aWindow) {
+      return this._windows[aWindow.__SSi]._closedTabs.length;
+    }
+
+    if (!DyingWindowCache.has(aWindow)) {
+      throw Components.Exception("Window is not tracked", Cr.NS_ERROR_INVALID_ARG);
+    }
+
+    return DyingWindowCache.get(aWindow)._closedTabs.length;
+  },
+
+  getClosedTabData: function ssi_getClosedTabData(aWindow, aAsString = true) {
+    if ("__SSi" in aWindow) {
+      return aAsString ?
+        JSON.stringify(this._windows[aWindow.__SSi]._closedTabs) :
+        Cu.cloneInto(this._windows[aWindow.__SSi]._closedTabs, {});
+    }
+
+    if (!DyingWindowCache.has(aWindow)) {
+      throw Components.Exception("Window is not tracked", Cr.NS_ERROR_INVALID_ARG);
+    }
+
+    let data = DyingWindowCache.get(aWindow);
+    return aAsString ? JSON.stringify(data._closedTabs) : Cu.cloneInto(data._closedTabs, {});
+  },
+
+  undoCloseTab: function ssi_undoCloseTab(aWindow, aIndex) {
+    if (!aWindow.__SSi) {
+      throw Components.Exception("Window is not tracked", Cr.NS_ERROR_INVALID_ARG);
+    }
+
+    var closedTabs = this._windows[aWindow.__SSi]._closedTabs;
+
+    // default to the most-recently closed tab
+    aIndex = aIndex || 0;
+    if (!(aIndex in closedTabs)) {
+      throw Components.Exception("Invalid index: not in the closed tabs", Cr.NS_ERROR_INVALID_ARG);
+    }
+
+    // fetch the data of closed tab, while removing it from the array
+    let {state, pos} = this.removeClosedTabData(closedTabs, aIndex);
+
+    // create a new tab
+    let tabbrowser = aWindow.gBrowser;
+    let tab = tabbrowser.selectedTab = tabbrowser.addTab(null, state);
+
+    // restore tab content
+    this.restoreTab(tab, state);
+
+    // restore the tab's position
+    tabbrowser.moveTabTo(tab, pos);
+
+    // focus the tab's content area (bug 342432)
+    tab.linkedBrowser.focus();
+
+    return tab;
+  },
+
+  forgetClosedTab: function ssi_forgetClosedTab(aWindow, aIndex) {
+    if (!aWindow.__SSi) {
+      throw Components.Exception("Window is not tracked", Cr.NS_ERROR_INVALID_ARG);
+    }
+
+    var closedTabs = this._windows[aWindow.__SSi]._closedTabs;
+
+    // default to the most-recently closed tab
+    aIndex = aIndex || 0;
+    if (!(aIndex in closedTabs)) {
+      throw Components.Exception("Invalid index: not in the closed tabs", Cr.NS_ERROR_INVALID_ARG);
+    }
+
+    // remove closed tab from the array
+    this.removeClosedTabData(closedTabs, aIndex);
+  },
+
+  getClosedWindowCount: function ssi_getClosedWindowCount() {
+    return this._closedWindows.length;
+  },
+
+  getClosedWindowData: function ssi_getClosedWindowData(aAsString = true) {
+    return aAsString ? JSON.stringify(this._closedWindows) : Cu.cloneInto(this._closedWindows, {});
+  },
+
+  undoCloseWindow: function ssi_undoCloseWindow(aIndex) {
+    if (!(aIndex in this._closedWindows)) {
+      throw Components.Exception("Invalid index: not in the closed windows", Cr.NS_ERROR_INVALID_ARG);
+    }
+
+    // reopen the window
+    let state = { windows: this._closedWindows.splice(aIndex, 1) };
+    delete state.windows[0].closedAt; // Window is now open.
+
+    let window = this._openWindowWithState(state);
+    this.windowToFocus = window;
+    return window;
+  },
+
+  forgetClosedWindow: function ssi_forgetClosedWindow(aIndex) {
+    // default to the most-recently closed window
+    aIndex = aIndex || 0;
+    if (!(aIndex in this._closedWindows)) {
+      throw Components.Exception("Invalid index: not in the closed windows", Cr.NS_ERROR_INVALID_ARG);
+    }
+
+    // remove closed window from the array
+    let winData = this._closedWindows[aIndex];
+    this._closedWindows.splice(aIndex, 1);
+    this._saveableClosedWindowData.delete(winData);
+  },
+
+  getWindowValue: function ssi_getWindowValue(aWindow, aKey) {
+    if ("__SSi" in aWindow) {
+      var data = this._windows[aWindow.__SSi].extData || {};
+      return data[aKey] || "";
+    }
+
+    if (DyingWindowCache.has(aWindow)) {
+      let data = DyingWindowCache.get(aWindow).extData || {};
+      return data[aKey] || "";
+    }
+
+    throw Components.Exception("Window is not tracked", Cr.NS_ERROR_INVALID_ARG);
+  },
+
+  setWindowValue: function ssi_setWindowValue(aWindow, aKey, aStringValue) {
+    if (typeof aStringValue != "string") {
+      throw new TypeError("setWindowValue only accepts string values");
+    }
+
+    if (!("__SSi" in aWindow)) {
+      throw Components.Exception("Window is not tracked", Cr.NS_ERROR_INVALID_ARG);
+    }
+    if (!this._windows[aWindow.__SSi].extData) {
+      this._windows[aWindow.__SSi].extData = {};
+    }
+    this._windows[aWindow.__SSi].extData[aKey] = aStringValue;
+    this.saveStateDelayed(aWindow);
+  },
+
+  deleteWindowValue: function ssi_deleteWindowValue(aWindow, aKey) {
+    if (aWindow.__SSi && this._windows[aWindow.__SSi].extData &&
+        this._windows[aWindow.__SSi].extData[aKey])
+      delete this._windows[aWindow.__SSi].extData[aKey];
+    this.saveStateDelayed(aWindow);
+  },
+
+  getTabValue: function ssi_getTabValue(aTab, aKey) {
+    return (aTab.__SS_extdata || {})[aKey] || "";
+  },
+
+  setTabValue: function ssi_setTabValue(aTab, aKey, aStringValue) {
+    if (typeof aStringValue != "string") {
+      throw new TypeError("setTabValue only accepts string values");
+    }
+
+    // If the tab hasn't been restored, then set the data there, otherwise we
+    // could lose newly added data.
+    if (!aTab.__SS_extdata) {
+      aTab.__SS_extdata = {};
+    }
+
+    aTab.__SS_extdata[aKey] = aStringValue;
+    this.saveStateDelayed(aTab.ownerGlobal);
+  },
+
+  deleteTabValue: function ssi_deleteTabValue(aTab, aKey) {
+    if (aTab.__SS_extdata && aKey in aTab.__SS_extdata) {
+      delete aTab.__SS_extdata[aKey];
+      this.saveStateDelayed(aTab.ownerGlobal);
+    }
+  },
+
+  getGlobalValue: function ssi_getGlobalValue(aKey) {
+    return this._globalState.get(aKey);
+  },
+
+  setGlobalValue: function ssi_setGlobalValue(aKey, aStringValue) {
+    if (typeof aStringValue != "string") {
+      throw new TypeError("setGlobalValue only accepts string values");
+    }
+
+    this._globalState.set(aKey, aStringValue);
+    this.saveStateDelayed();
+  },
+
+  deleteGlobalValue: function ssi_deleteGlobalValue(aKey) {
+    this._globalState.delete(aKey);
+    this.saveStateDelayed();
+  },
+
+  persistTabAttribute: function ssi_persistTabAttribute(aName) {
+    if (TabAttributes.persist(aName)) {
+      this.saveStateDelayed();
+    }
+  },
+
+
+  /**
+   * Undoes the closing of a tab or window which corresponds
+   * to the closedId passed in.
+   *
+   * @param aClosedId
+   *        The closedId of the tab or window
+   *
+   * @returns a tab or window object
+   */
+  undoCloseById(aClosedId) {
+    // Check for a window first.
+    for (let i = 0, l = this._closedWindows.length; i < l; i++) {
+      if (this._closedWindows[i].closedId == aClosedId) {
+        return this.undoCloseWindow(i);
+      }
+    }
+
+    // Check for a tab.
+    let windowsEnum = Services.wm.getEnumerator("navigator:browser");
+    while (windowsEnum.hasMoreElements()) {
+      let window = windowsEnum.getNext();
+      let windowState = this._windows[window.__SSi];
+      if (windowState) {
+        for (let j = 0, l = windowState._closedTabs.length; j < l; j++) {
+          if (windowState._closedTabs[j].closedId == aClosedId) {
+            return this.undoCloseTab(window, j);
+          }
+        }
+      }
+    }
+
+    // Neither a tab nor a window was found, return undefined and let the caller decide what to do about it.
+    return undefined;
+  },
+
+  /**
+   * Restores the session state stored in LastSession. This will attempt
+   * to merge data into the current session. If a window was opened at startup
+   * with pinned tab(s), then the remaining data from the previous session for
+   * that window will be opened into that window. Otherwise new windows will
+   * be opened.
+   */
+  restoreLastSession: function ssi_restoreLastSession() {
+    // Use the public getter since it also checks PB mode
+    if (!this.canRestoreLastSession) {
+      throw Components.Exception("Last session can not be restored");
+    }
+
+    Services.obs.notifyObservers(null, NOTIFY_INITIATING_MANUAL_RESTORE, "");
+
+    // First collect each window with its id...
+    let windows = {};
+    this._forEachBrowserWindow(function(aWindow) {
+      if (aWindow.__SS_lastSessionWindowID)
+        windows[aWindow.__SS_lastSessionWindowID] = aWindow;
+    });
+
+    let lastSessionState = LastSession.getState();
+
+    // This shouldn't ever be the case...
+    if (!lastSessionState.windows.length) {
+      throw Components.Exception("lastSessionState has no windows", Cr.NS_ERROR_UNEXPECTED);
+    }
+
+    // We're technically doing a restore, so set things up so we send the
+    // notification when we're done. We want to send "sessionstore-browser-state-restored".
+    this._restoreCount = lastSessionState.windows.length;
+    this._browserSetState = true;
+
+    // We want to re-use the last opened window instead of opening a new one in
+    // the case where it's "empty" and not associated with a window in the session.
+    // We will do more processing via _prepWindowToRestoreInto if we need to use
+    // the lastWindow.
+    let lastWindow = this._getMostRecentBrowserWindow();
+    let canUseLastWindow = lastWindow &&
+                           !lastWindow.__SS_lastSessionWindowID;
+
+    // global data must be restored before restoreWindow is called so that
+    // it happens before observers are notified
+    this._globalState.setFromState(lastSessionState);
+
+    // Restore into windows or open new ones as needed.
+    for (let i = 0; i < lastSessionState.windows.length; i++) {
+      let winState = lastSessionState.windows[i];
+      let lastSessionWindowID = winState.__lastSessionWindowID;
+      // delete lastSessionWindowID so we don't add that to the window again
+      delete winState.__lastSessionWindowID;
+
+      // See if we can use an open window. First try one that is associated with
+      // the state we're trying to restore and then fallback to the last selected
+      // window.
+      let windowToUse = windows[lastSessionWindowID];
+      if (!windowToUse && canUseLastWindow) {
+        windowToUse = lastWindow;
+        canUseLastWindow = false;
+      }
+
+      let [canUseWindow, canOverwriteTabs] = this._prepWindowToRestoreInto(windowToUse);
+
+      // If there's a window already open that we can restore into, use that
+      if (canUseWindow) {
+        // Since we're not overwriting existing tabs, we want to merge _closedTabs,
+        // putting existing ones first. Then make sure we're respecting the max pref.
+        if (winState._closedTabs && winState._closedTabs.length) {
+          let curWinState = this._windows[windowToUse.__SSi];
+          curWinState._closedTabs = curWinState._closedTabs.concat(winState._closedTabs);
+          curWinState._closedTabs.splice(this._max_tabs_undo, curWinState._closedTabs.length);
+        }
+
+        // Restore into that window - pretend it's a followup since we'll already
+        // have a focused window.
+        //XXXzpao This is going to merge extData together (taking what was in
+        //        winState over what is in the window already.
+        let options = {overwriteTabs: canOverwriteTabs, isFollowUp: true};
+        this.restoreWindow(windowToUse, winState, options);
+      }
+      else {
+        this._openWindowWithState({ windows: [winState] });
+      }
+    }
+
+    // Merge closed windows from this session with ones from last session
+    if (lastSessionState._closedWindows) {
+      this._closedWindows = this._closedWindows.concat(lastSessionState._closedWindows);
+      this._capClosedWindows();
+    }
+
+    // Scratchpad
+    if (lastSessionState.scratchpads) {
+      ScratchpadManager.restoreSession(lastSessionState.scratchpads);
+    }
+
+    // The Browser Console
+    if (lastSessionState.browserConsole) {
+      HUDService.restoreBrowserConsoleSession();
+    }
+
+    // Set data that persists between sessions
+    this._recentCrashes = lastSessionState.session &&
+                          lastSessionState.session.recentCrashes || 0;
+
+    // Update the session start time using the restored session state.
+    this._updateSessionStartTime(lastSessionState);
+
+    LastSession.clear();
+  },
+
+  /**
+   * Revive a crashed tab and restore its state from before it crashed.
+   *
+   * @param aTab
+   *        A <xul:tab> linked to a crashed browser. This is a no-op if the
+   *        browser hasn't actually crashed, or is not associated with a tab.
+   *        This function will also throw if the browser happens to be remote.
+   */
+  reviveCrashedTab(aTab) {
+    if (!aTab) {
+      throw new Error("SessionStore.reviveCrashedTab expected a tab, but got null.");
+    }
+
+    let browser = aTab.linkedBrowser;
+    if (!this._crashedBrowsers.has(browser.permanentKey)) {
+      return;
+    }
+
+    // Sanity check - the browser to be revived should not be remote
+    // at this point.
+    if (browser.isRemoteBrowser) {
+      throw new Error("SessionStore.reviveCrashedTab: " +
+                      "Somehow a crashed browser is still remote.")
+    }
+
+    // We put the browser at about:blank in case the user is
+    // restoring tabs on demand. This way, the user won't see
+    // a flash of the about:tabcrashed page after selecting
+    // the revived tab.
+    aTab.removeAttribute("crashed");
+    browser.loadURI("about:blank", null, null);
+
+    let data = TabState.collect(aTab);
+    this.restoreTab(aTab, data, {
+      forceOnDemand: true,
+    });
+  },
+
+  /**
+   * Revive all crashed tabs and reset the crashed tabs count to 0.
+   */
+  reviveAllCrashedTabs() {
+    let windowsEnum = Services.wm.getEnumerator("navigator:browser");
+    while (windowsEnum.hasMoreElements()) {
+      let window = windowsEnum.getNext();
+      for (let tab of window.gBrowser.tabs) {
+        this.reviveCrashedTab(tab);
+      }
+    }
+  },
+
+  /**
+   * Navigate the given |tab| by first collecting its current state and then
+   * either changing only the index of the currently shown history entry,
+   * or restoring the exact same state again and passing the new URL to load
+   * in |loadArguments|. Use this method to seamlessly switch between pages
+   * loaded in the parent and pages loaded in the child process.
+   *
+   * This method might be called multiple times before it has finished
+   * flushing the browser tab. If that occurs, the loadArguments from
+   * the most recent call to navigateAndRestore will be used once the
+   * flush has finished.
+   */
+  navigateAndRestore(tab, loadArguments, historyIndex) {
+    let window = tab.ownerGlobal;
+    NS_ASSERT(window.__SSi, "tab's window must be tracked");
+    let browser = tab.linkedBrowser;
+
+    // Were we already waiting for a flush from a previous call to
+    // navigateAndRestore on this tab?
+    let alreadyRestoring =
+      this._remotenessChangingBrowsers.has(browser.permanentKey);
+
+    // Stash the most recent loadArguments in this WeakMap so that
+    // we know to use it when the TabStateFlusher.flush resolves.
+    this._remotenessChangingBrowsers.set(browser.permanentKey, loadArguments);
+
+    if (alreadyRestoring) {
+      // This tab was already being restored to run in the
+      // correct process. We're done here.
+      return;
+    }
+
+    // Set tab title to "Connecting..." and start the throbber to pretend we're
+    // doing something while actually waiting for data from the frame script.
+    window.gBrowser.setTabTitleLoading(tab);
+    tab.setAttribute("busy", "true");
+
+    // Flush to get the latest tab state.
+    TabStateFlusher.flush(browser).then(() => {
+      // loadArguments might have been overwritten by multiple calls
+      // to navigateAndRestore while we waited for the tab to flush,
+      // so we use the most recently stored one.
+      let recentLoadArguments =
+        this._remotenessChangingBrowsers.get(browser.permanentKey);
+      this._remotenessChangingBrowsers.delete(browser.permanentKey);
+
+      // The tab might have been closed/gone in the meantime.
+      if (tab.closing || !tab.linkedBrowser) {
+        return;
+      }
+
+      let window = tab.ownerGlobal;
+
+      // The tab or its window might be gone.
+      if (!window || !window.__SSi || window.closed) {
+        return;
+      }
+
+      let tabState = TabState.clone(tab);
+      let options = {
+        restoreImmediately: true,
+        // We want to make sure that this information is passed to restoreTab
+        // whether or not a historyIndex is passed in. Thus, we extract it from
+        // the loadArguments.
+        reloadInFreshProcess: !!recentLoadArguments.reloadInFreshProcess,
+      };
+
+      if (historyIndex >= 0) {
+        tabState.index = historyIndex + 1;
+        tabState.index = Math.max(1, Math.min(tabState.index, tabState.entries.length));
+      } else {
+        options.loadArguments = recentLoadArguments;
+      }
+
+      // Need to reset restoring tabs.
+      if (tab.linkedBrowser.__SS_restoreState) {
+        this._resetLocalTabRestoringState(tab);
+      }
+
+      // Restore the state into the tab.
+      this.restoreTab(tab, tabState, options);
+    });
+
+    tab.linkedBrowser.__SS_restoreState = TAB_STATE_WILL_RESTORE;
+  },
+
+  /**
+   * Retrieves the latest session history information for a tab. The cached data
+   * is returned immediately, but a callback may be provided that supplies
+   * up-to-date data when or if it is available. The callback is passed a single
+   * argument with data in the same format as the return value.
+   *
+   * @param tab tab to retrieve the session history for
+   * @param updatedCallback function to call with updated data as the single argument
+   * @returns a object containing 'index' specifying the current index, and an
+   * array 'entries' containing an object for each history item.
+   */
+  getSessionHistory(tab, updatedCallback) {
+    if (updatedCallback) {
+      TabStateFlusher.flush(tab.linkedBrowser).then(() => {
+        let sessionHistory = this.getSessionHistory(tab);
+        if (sessionHistory) {
+          updatedCallback(sessionHistory);
+        }
+      });
+    }
+
+    // Don't continue if the tab was closed before TabStateFlusher.flush resolves.
+    if (tab.linkedBrowser) {
+      let tabState = TabState.collect(tab);
+      return { index: tabState.index - 1, entries: tabState.entries }
+    }
+  },
+
+  /**
+   * See if aWindow is usable for use when restoring a previous session via
+   * restoreLastSession. If usable, prepare it for use.
+   *
+   * @param aWindow
+   *        the window to inspect & prepare
+   * @returns [canUseWindow, canOverwriteTabs]
+   *          canUseWindow: can the window be used to restore into
+   *          canOverwriteTabs: all of the current tabs are home pages and we
+   *                            can overwrite them
+   */
+  _prepWindowToRestoreInto: function ssi_prepWindowToRestoreInto(aWindow) {
+    if (!aWindow)
+      return [false, false];
+
+    // We might be able to overwrite the existing tabs instead of just adding
+    // the previous session's tabs to the end. This will be set if possible.
+    let canOverwriteTabs = false;
+
+    // Look at the open tabs in comparison to home pages. If all the tabs are
+    // home pages then we'll end up overwriting all of them. Otherwise we'll
+    // just close the tabs that match home pages. Tabs with the about:blank
+    // URI will always be overwritten.
+    let homePages = ["about:blank"];
+    let removableTabs = [];
+    let tabbrowser = aWindow.gBrowser;
+    let normalTabsLen = tabbrowser.tabs.length - tabbrowser._numPinnedTabs;
+    let startupPref = this._prefBranch.getIntPref("startup.page");
+    if (startupPref == 1)
+      homePages = homePages.concat(aWindow.gHomeButton.getHomePage().split("|"));
+
+    for (let i = tabbrowser._numPinnedTabs; i < tabbrowser.tabs.length; i++) {
+      let tab = tabbrowser.tabs[i];
+      if (homePages.indexOf(tab.linkedBrowser.currentURI.spec) != -1) {
+        removableTabs.push(tab);
+      }
+    }
+
+    if (tabbrowser.tabs.length == removableTabs.length) {
+      canOverwriteTabs = true;
+    }
+    else {
+      // If we're not overwriting all of the tabs, then close the home tabs.
+      for (let i = removableTabs.length - 1; i >= 0; i--) {
+        tabbrowser.removeTab(removableTabs.pop(), { animate: false });
+      }
+    }
+
+    return [true, canOverwriteTabs];
+  },
+
+  /* ........ Saving Functionality .............. */
+
+  /**
+   * Store window dimensions, visibility, sidebar
+   * @param aWindow
+   *        Window reference
+   */
+  _updateWindowFeatures: function ssi_updateWindowFeatures(aWindow) {
+    var winData = this._windows[aWindow.__SSi];
+
+    WINDOW_ATTRIBUTES.forEach(function(aAttr) {
+      winData[aAttr] = this._getWindowDimension(aWindow, aAttr);
+    }, this);
+
+    var hidden = WINDOW_HIDEABLE_FEATURES.filter(function(aItem) {
+      return aWindow[aItem] && !aWindow[aItem].visible;
+    });
+    if (hidden.length != 0)
+      winData.hidden = hidden.join(",");
+    else if (winData.hidden)
+      delete winData.hidden;
+
+    var sidebar = aWindow.document.getElementById("sidebar-box").getAttribute("sidebarcommand");
+    if (sidebar)
+      winData.sidebar = sidebar;
+    else if (winData.sidebar)
+      delete winData.sidebar;
+  },
+
+  /**
+   * gather session data as object
+   * @param aUpdateAll
+   *        Bool update all windows
+   * @returns object
+   */
+  getCurrentState: function (aUpdateAll) {
+    this._handleClosedWindows();
+
+    var activeWindow = this._getMostRecentBrowserWindow();
+
+    TelemetryStopwatch.start("FX_SESSION_RESTORE_COLLECT_ALL_WINDOWS_DATA_MS");
+    if (RunState.isRunning) {
+      // update the data for all windows with activities since the last save operation
+      this._forEachBrowserWindow(function(aWindow) {
+        if (!this._isWindowLoaded(aWindow)) // window data is still in _statesToRestore
+          return;
+        if (aUpdateAll || DirtyWindows.has(aWindow) || aWindow == activeWindow) {
+          this._collectWindowData(aWindow);
+        }
+        else { // always update the window features (whose change alone never triggers a save operation)
+          this._updateWindowFeatures(aWindow);
+        }
+      });
+      DirtyWindows.clear();
+    }
+    TelemetryStopwatch.finish("FX_SESSION_RESTORE_COLLECT_ALL_WINDOWS_DATA_MS");
+
+    // An array that at the end will hold all current window data.
+    var total = [];
+    // The ids of all windows contained in 'total' in the same order.
+    var ids = [];
+    // The number of window that are _not_ popups.
+    var nonPopupCount = 0;
+    var ix;
+
+    // collect the data for all windows
+    for (ix in this._windows) {
+      if (this._windows[ix]._restoring) // window data is still in _statesToRestore
+        continue;
+      total.push(this._windows[ix]);
+      ids.push(ix);
+      if (!this._windows[ix].isPopup)
+        nonPopupCount++;
+    }
+
+    TelemetryStopwatch.start("FX_SESSION_RESTORE_COLLECT_COOKIES_MS");
+    SessionCookies.update(total);
+    TelemetryStopwatch.finish("FX_SESSION_RESTORE_COLLECT_COOKIES_MS");
+
+    // collect the data for all windows yet to be restored
+    for (ix in this._statesToRestore) {
+      for (let winData of this._statesToRestore[ix].windows) {
+        total.push(winData);
+        if (!winData.isPopup)
+          nonPopupCount++;
+      }
+    }
+
+    // shallow copy this._closedWindows to preserve current state
+    let lastClosedWindowsCopy = this._closedWindows.slice();
+
+    if (AppConstants.platform != "macosx") {
+      // If no non-popup browser window remains open, return the state of the last
+      // closed window(s). We only want to do this when we're actually "ending"
+      // the session.
+      //XXXzpao We should do this for _restoreLastWindow == true, but that has
+      //        its own check for popups. c.f. bug 597619
+      if (nonPopupCount == 0 && lastClosedWindowsCopy.length > 0 &&
+          RunState.isQuitting) {
+        // prepend the last non-popup browser window, so that if the user loads more tabs
+        // at startup we don't accidentally add them to a popup window
+        do {
+          total.unshift(lastClosedWindowsCopy.shift())
+        } while (total[0].isPopup && lastClosedWindowsCopy.length > 0)
+      }
+    }
+
+    if (activeWindow) {
+      this.activeWindowSSiCache = activeWindow.__SSi || "";
+    }
+    ix = ids.indexOf(this.activeWindowSSiCache);
+    // We don't want to restore focus to a minimized window or a window which had all its
+    // tabs stripped out (doesn't exist).
+    if (ix != -1 && total[ix] && total[ix].sizemode == "minimized")
+      ix = -1;
+
+    let session = {
+      lastUpdate: Date.now(),
+      startTime: this._sessionStartTime,
+      recentCrashes: this._recentCrashes
+    };
+
+    let state = {
+      version: ["sessionrestore", FORMAT_VERSION],
+      windows: total,
+      selectedWindow: ix + 1,
+      _closedWindows: lastClosedWindowsCopy,
+      session: session,
+      global: this._globalState.getState()
+    };
+
+    // Scratchpad
+    if (Cu.isModuleLoaded("resource://devtools/client/scratchpad/scratchpad-manager.jsm")) {
+      // get open Scratchpad window states too
+      let scratchpads = ScratchpadManager.getSessionState();
+      if (scratchpads && scratchpads.length) {
+        state.scratchpads = scratchpads;
+      }
+    }
+
+    // The Browser Console
+    state.browserConsole = HUDService.getBrowserConsoleSessionState();
+
+    // Persist the last session if we deferred restoring it
+    if (LastSession.canRestore) {
+      state.lastSessionState = LastSession.getState();
+    }
+
+    // If we were called by the SessionSaver and started with only a private
+    // window we want to pass the deferred initial state to not lose the
+    // previous session.
+    if (this._deferredInitialState) {
+      state.deferredInitialState = this._deferredInitialState;
+    }
+
+    return state;
+  },
+
+  /**
+   * serialize session data for a window
+   * @param aWindow
+   *        Window reference
+   * @returns string
+   */
+  _getWindowState: function ssi_getWindowState(aWindow) {
+    if (!this._isWindowLoaded(aWindow))
+      return this._statesToRestore[aWindow.__SS_restoreID];
+
+    if (RunState.isRunning) {
+      this._collectWindowData(aWindow);
+    }
+
+    let windows = [this._windows[aWindow.__SSi]];
+    SessionCookies.update(windows);
+
+    return { windows: windows };
+  },
+
+  /**
+   * Gathers data about a window and its tabs, and updates its
+   * entry in this._windows.
+   *
+   * @param aWindow
+   *        Window references.
+   * @returns a Map mapping the browser tabs from aWindow to the tab
+   *          entry that was put into the window data in this._windows.
+   */
+  _collectWindowData: function ssi_collectWindowData(aWindow) {
+    let tabMap = new Map();
+
+    if (!this._isWindowLoaded(aWindow))
+      return tabMap;
+
+    let tabbrowser = aWindow.gBrowser;
+    let tabs = tabbrowser.tabs;
+    let winData = this._windows[aWindow.__SSi];
+    let tabsData = winData.tabs = [];
+
+    // update the internal state data for this window
+    for (let tab of tabs) {
+      let tabData = TabState.collect(tab);
+      tabMap.set(tab, tabData);
+      tabsData.push(tabData);
+    }
+    winData.selected = tabbrowser.mTabBox.selectedIndex + 1;
+
+    this._updateWindowFeatures(aWindow);
+
+    // Make sure we keep __SS_lastSessionWindowID around for cases like entering
+    // or leaving PB mode.
+    if (aWindow.__SS_lastSessionWindowID)
+      this._windows[aWindow.__SSi].__lastSessionWindowID =
+        aWindow.__SS_lastSessionWindowID;
+
+    DirtyWindows.remove(aWindow);
+    return tabMap;
+  },
+
+  /* ........ Restoring Functionality .............. */
+
+  /**
+   * restore features to a single window
+   * @param aWindow
+   *        Window reference to the window to use for restoration
+   * @param winData
+   *        JS object
+   * @param aOptions
+   *        {overwriteTabs: true} to overwrite existing tabs w/ new ones
+   *        {isFollowUp: true} if this is not the restoration of the 1st window
+   *        {firstWindow: true} if this is the first non-private window we're
+   *                            restoring in this session, that might open an
+   *                            external link as well
+   */
+  restoreWindow: function ssi_restoreWindow(aWindow, winData, aOptions = {}) {
+    let overwriteTabs = aOptions && aOptions.overwriteTabs;
+    let isFollowUp = aOptions && aOptions.isFollowUp;
+    let firstWindow = aOptions && aOptions.firstWindow;
+
+    if (isFollowUp) {
+      this.windowToFocus = aWindow;
+    }
+
+    // initialize window if necessary
+    if (aWindow && (!aWindow.__SSi || !this._windows[aWindow.__SSi]))
+      this.onLoad(aWindow);
+
+    TelemetryStopwatch.start("FX_SESSION_RESTORE_RESTORE_WINDOW_MS");
+
+    // We're not returning from this before we end up calling restoreTabs
+    // for this window, so make sure we send the SSWindowStateBusy event.
+    this._setWindowStateBusy(aWindow);
+
+    if (!winData.tabs) {
+      winData.tabs = [];
+    }
+
+    // don't restore a single blank tab when we've had an external
+    // URL passed in for loading at startup (cf. bug 357419)
+    else if (firstWindow && !overwriteTabs && winData.tabs.length == 1 &&
+             (!winData.tabs[0].entries || winData.tabs[0].entries.length == 0)) {
+      winData.tabs = [];
+    }
+
+    var tabbrowser = aWindow.gBrowser;
+    var openTabCount = overwriteTabs ? tabbrowser.browsers.length : -1;
+    var newTabCount = winData.tabs.length;
+    var tabs = [];
+
+    // disable smooth scrolling while adding, moving, removing and selecting tabs
+    var tabstrip = tabbrowser.tabContainer.mTabstrip;
+    var smoothScroll = tabstrip.smoothScroll;
+    tabstrip.smoothScroll = false;
+
+    // unpin all tabs to ensure they are not reordered in the next loop
+    if (overwriteTabs) {
+      for (let t = tabbrowser._numPinnedTabs - 1; t > -1; t--)
+        tabbrowser.unpinTab(tabbrowser.tabs[t]);
+    }
+
+    // We need to keep track of the initially open tabs so that they
+    // can be moved to the end of the restored tabs.
+    let initialTabs = [];
+    if (!overwriteTabs && firstWindow) {
+      initialTabs = Array.slice(tabbrowser.tabs);
+    }
+
+    // make sure that the selected tab won't be closed in order to
+    // prevent unnecessary flickering
+    if (overwriteTabs && tabbrowser.selectedTab._tPos >= newTabCount)
+      tabbrowser.moveTabTo(tabbrowser.selectedTab, newTabCount - 1);
+
+    let numVisibleTabs = 0;
+
+    for (var t = 0; t < newTabCount; t++) {
+      // When trying to restore into existing tab, we also take the userContextId
+      // into account if present.
+      let userContextId = winData.tabs[t].userContextId;
+      let reuseExisting = t < openTabCount &&
+                          (tabbrowser.tabs[t].getAttribute("usercontextid") == (userContextId || ""));
+      // If the tab is pinned, then we'll be loading it right away, and
+      // there's no need to cause a remoteness flip by loading it initially
+      // non-remote.
+      let forceNotRemote = !winData.tabs[t].pinned;
+      let tab = reuseExisting ? tabbrowser.tabs[t] :
+                                tabbrowser.addTab("about:blank",
+                                                  {skipAnimation: true,
+                                                   forceNotRemote,
+                                                   userContextId});
+
+      // If we inserted a new tab because the userContextId didn't match with the
+      // open tab, even though `t < openTabCount`, we need to remove that open tab
+      // and put the newly added tab in its place.
+      if (!reuseExisting && t < openTabCount) {
+        tabbrowser.removeTab(tabbrowser.tabs[t]);
+        tabbrowser.moveTabTo(tab, t);
+      }
+
+      tabs.push(tab);
+
+      if (winData.tabs[t].pinned)
+        tabbrowser.pinTab(tabs[t]);
+
+      if (winData.tabs[t].hidden) {
+        tabbrowser.hideTab(tabs[t]);
+      }
+      else {
+        tabbrowser.showTab(tabs[t]);
+        numVisibleTabs++;
+      }
+
+      if (!!winData.tabs[t].muted != tabs[t].linkedBrowser.audioMuted) {
+        tabs[t].toggleMuteAudio(winData.tabs[t].muteReason);
+      }
+    }
+
+    if (!overwriteTabs && firstWindow) {
+      // Move the originally open tabs to the end
+      let endPosition = tabbrowser.tabs.length - 1;
+      for (let i = 0; i < initialTabs.length; i++) {
+        tabbrowser.moveTabTo(initialTabs[i], endPosition);
+      }
+    }
+
+    // if all tabs to be restored are hidden, make the first one visible
+    if (!numVisibleTabs && winData.tabs.length) {
+      winData.tabs[0].hidden = false;
+      tabbrowser.showTab(tabs[0]);
+    }
+
+    // If overwriting tabs, we want to reset each tab's "restoring" state. Since
+    // we're overwriting those tabs, they should no longer be restoring. The
+    // tabs will be rebuilt and marked if they need to be restored after loading
+    // state (in restoreTabs).
+    if (overwriteTabs) {
+      for (let i = 0; i < tabbrowser.tabs.length; i++) {
+        let tab = tabbrowser.tabs[i];
+        if (tabbrowser.browsers[i].__SS_restoreState)
+          this._resetTabRestoringState(tab);
+      }
+    }
+
+    // We want to correlate the window with data from the last session, so
+    // assign another id if we have one. Otherwise clear so we don't do
+    // anything with it.
+    delete aWindow.__SS_lastSessionWindowID;
+    if (winData.__lastSessionWindowID)
+      aWindow.__SS_lastSessionWindowID = winData.__lastSessionWindowID;
+
+    // when overwriting tabs, remove all superflous ones
+    if (overwriteTabs && newTabCount < openTabCount) {
+      Array.slice(tabbrowser.tabs, newTabCount, openTabCount)
+           .forEach(tabbrowser.removeTab, tabbrowser);
+    }
+
+    if (overwriteTabs) {
+      this.restoreWindowFeatures(aWindow, winData);
+      delete this._windows[aWindow.__SSi].extData;
+    }
+    if (winData.cookies) {
+      SessionCookies.restore(winData.cookies);
+    }
+    if (winData.extData) {
+      if (!this._windows[aWindow.__SSi].extData) {
+        this._windows[aWindow.__SSi].extData = {};
+      }
+      for (var key in winData.extData) {
+        this._windows[aWindow.__SSi].extData[key] = winData.extData[key];
+      }
+    }
+
+    let newClosedTabsData = winData._closedTabs || [];
+
+    if (overwriteTabs || firstWindow) {
+      // Overwrite existing closed tabs data when overwriteTabs=true
+      // or we're the first window to be restored.
+      this._windows[aWindow.__SSi]._closedTabs = newClosedTabsData;
+    } else if (this._max_tabs_undo > 0) {
+      // If we merge tabs, we also want to merge closed tabs data. We'll assume
+      // the restored tabs were closed more recently and append the current list
+      // of closed tabs to the new one...
+      newClosedTabsData =
+        newClosedTabsData.concat(this._windows[aWindow.__SSi]._closedTabs);
+
+      // ... and make sure that we don't exceed the max number of closed tabs
+      // we can restore.
+      this._windows[aWindow.__SSi]._closedTabs =
+        newClosedTabsData.slice(0, this._max_tabs_undo);
+    }
+
+    // Restore tabs, if any.
+    if (winData.tabs.length) {
+      this.restoreTabs(aWindow, tabs, winData.tabs,
+        (overwriteTabs ? (parseInt(winData.selected || "1")) : 0));
+    }
+
+    // set smoothScroll back to the original value
+    tabstrip.smoothScroll = smoothScroll;
+
+    TelemetryStopwatch.finish("FX_SESSION_RESTORE_RESTORE_WINDOW_MS");
+
+    this._setWindowStateReady(aWindow);
+
+    this._sendWindowRestoredNotification(aWindow);
+
+    Services.obs.notifyObservers(aWindow, NOTIFY_SINGLE_WINDOW_RESTORED, "");
+
+    this._sendRestoreCompletedNotifications();
+  },
+
+  /**
+   * Restore multiple windows using the provided state.
+   * @param aWindow
+   *        Window reference to the first window to use for restoration.
+   *        Additionally required windows will be opened.
+   * @param aState
+   *        JS object or JSON string
+   * @param aOptions
+   *        {overwriteTabs: true} to overwrite existing tabs w/ new ones
+   *        {isFollowUp: true} if this is not the restoration of the 1st window
+   *        {firstWindow: true} if this is the first non-private window we're
+   *                            restoring in this session, that might open an
+   *                            external link as well
+   */
+  restoreWindows: function ssi_restoreWindows(aWindow, aState, aOptions = {}) {
+    let isFollowUp = aOptions && aOptions.isFollowUp;
+
+    if (isFollowUp) {
+      this.windowToFocus = aWindow;
+    }
+
+    // initialize window if necessary
+    if (aWindow && (!aWindow.__SSi || !this._windows[aWindow.__SSi]))
+      this.onLoad(aWindow);
+
+    let root;
+    try {
+      root = (typeof aState == "string") ? JSON.parse(aState) : aState;
+    }
+    catch (ex) { // invalid state object - don't restore anything
+      debug(ex);
+      this._sendRestoreCompletedNotifications();
+      return;
+    }
+
+    // Restore closed windows if any.
+    if (root._closedWindows) {
+      this._closedWindows = root._closedWindows;
+    }
+
+    // We're done here if there are no windows.
+    if (!root.windows || !root.windows.length) {
+      this._sendRestoreCompletedNotifications();
+      return;
+    }
+
+    if (!root.selectedWindow || root.selectedWindow > root.windows.length) {
+      root.selectedWindow = 0;
+    }
+
+    // open new windows for all further window entries of a multi-window session
+    // (unless they don't contain any tab data)
+    let winData;
+    for (var w = 1; w < root.windows.length; w++) {
+      winData = root.windows[w];
+      if (winData && winData.tabs && winData.tabs[0]) {
+        var window = this._openWindowWithState({ windows: [winData] });
+        if (w == root.selectedWindow - 1) {
+          this.windowToFocus = window;
+        }
+      }
+    }
+
+    this.restoreWindow(aWindow, root.windows[0], aOptions);
+
+    // Scratchpad
+    if (aState.scratchpads) {
+      ScratchpadManager.restoreSession(aState.scratchpads);
+    }
+
+    // The Browser Console
+    if (aState.browserConsole) {
+      HUDService.restoreBrowserConsoleSession();
+    }
+  },
+
+  /**
+   * Manage history restoration for a window
+   * @param aWindow
+   *        Window to restore the tabs into
+   * @param aTabs
+   *        Array of tab references
+   * @param aTabData
+   *        Array of tab data
+   * @param aSelectTab
+   *        Index of the tab to select. This is a 1-based index where "1"
+   *        indicates the first tab should be selected, and "0" indicates that
+   *        the currently selected tab will not be changed.
+   */
+  restoreTabs(aWindow, aTabs, aTabData, aSelectTab) {
+    var tabbrowser = aWindow.gBrowser;
+
+    if (!this._isWindowLoaded(aWindow)) {
+      // from now on, the data will come from the actual window
+      delete this._statesToRestore[aWindow.__SS_restoreID];
+      delete aWindow.__SS_restoreID;
+      delete this._windows[aWindow.__SSi]._restoring;
+    }
+
+    let numTabsToRestore = aTabs.length;
+    let numTabsInWindow = tabbrowser.tabs.length;
+    let tabsDataArray = this._windows[aWindow.__SSi].tabs;
+
+    // Update the window state in case we shut down without being notified.
+    // Individual tab states will be taken care of by restoreTab() below.
+    if (numTabsInWindow == numTabsToRestore) {
+      // Remove all previous tab data.
+      tabsDataArray.length = 0;
+    } else {
+      // Remove all previous tab data except tabs that should not be overriden.
+      tabsDataArray.splice(numTabsInWindow - numTabsToRestore);
+    }
+
+    // Let the tab data array have the right number of slots.
+    tabsDataArray.length = numTabsInWindow;
+
+    // If provided, set the selected tab.
+    if (aSelectTab > 0 && aSelectTab <= aTabs.length) {
+      tabbrowser.selectedTab = aTabs[aSelectTab - 1];
+
+      // Update the window state in case we shut down without being notified.
+      this._windows[aWindow.__SSi].selected = aSelectTab;
+    }
+
+    // Restore all tabs.
+    for (let t = 0; t < aTabs.length; t++) {
+      this.restoreTab(aTabs[t], aTabData[t]);
+    }
+  },
+
+  // Restores the given tab state for a given tab.
+  restoreTab(tab, tabData, options = {}) {
+    NS_ASSERT(!tab.linkedBrowser.__SS_restoreState,
+              "must reset tab before calling restoreTab()");
+
+    let restoreImmediately = options.restoreImmediately;
+    let loadArguments = options.loadArguments;
+    let browser = tab.linkedBrowser;
+    let window = tab.ownerGlobal;
+    let tabbrowser = window.gBrowser;
+    let forceOnDemand = options.forceOnDemand;
+    let reloadInFreshProcess = options.reloadInFreshProcess;
+
+    let willRestoreImmediately = restoreImmediately ||
+                                 tabbrowser.selectedBrowser == browser ||
+                                 loadArguments;
+
+    if (!willRestoreImmediately && !forceOnDemand) {
+      TabRestoreQueue.add(tab);
+    }
+
+    this._maybeUpdateBrowserRemoteness({ tabbrowser, tab,
+                                         willRestoreImmediately });
+
+    // Increase the busy state counter before modifying the tab.
+    this._setWindowStateBusy(window);
+
+    // It's important to set the window state to dirty so that
+    // we collect their data for the first time when saving state.
+    DirtyWindows.add(window);
+
+    // In case we didn't collect/receive data for any tabs yet we'll have to
+    // fill the array with at least empty tabData objects until |_tPos| or
+    // we'll end up with |null| entries.
+    for (let otherTab of Array.slice(tabbrowser.tabs, 0, tab._tPos)) {
+      let emptyState = {entries: [], lastAccessed: otherTab.lastAccessed};
+      this._windows[window.__SSi].tabs.push(emptyState);
+    }
+
+    // Update the tab state in case we shut down without being notified.
+    this._windows[window.__SSi].tabs[tab._tPos] = tabData;
+
+    // Prepare the tab so that it can be properly restored. We'll pin/unpin
+    // and show/hide tabs as necessary. We'll also attach a copy of the tab's
+    // data in case we close it before it's been restored.
+    if (tabData.pinned) {
+      tabbrowser.pinTab(tab);
+    } else {
+      tabbrowser.unpinTab(tab);
+    }
+
+    if (tabData.hidden) {
+      tabbrowser.hideTab(tab);
+    } else {
+      tabbrowser.showTab(tab);
+    }
+
+    if (!!tabData.muted != browser.audioMuted) {
+      tab.toggleMuteAudio(tabData.muteReason);
+    }
+
+    if (tabData.lastAccessed) {
+      tab.updateLastAccessed(tabData.lastAccessed);
+    }
+
+    if ("attributes" in tabData) {
+      // Ensure that we persist tab attributes restored from previous sessions.
+      Object.keys(tabData.attributes).forEach(a => TabAttributes.persist(a));
+    }
+
+    if (!tabData.entries) {
+      tabData.entries = [];
+    }
+    if (tabData.extData) {
+      tab.__SS_extdata = Cu.cloneInto(tabData.extData, {});
+    } else {
+      delete tab.__SS_extdata;
+    }
+
+    // Tab is now open.
+    delete tabData.closedAt;
+
+    // Ensure the index is in bounds.
+    let activeIndex = (tabData.index || tabData.entries.length) - 1;
+    activeIndex = Math.min(activeIndex, tabData.entries.length - 1);
+    activeIndex = Math.max(activeIndex, 0);
+
+    // Save the index in case we updated it above.
+    tabData.index = activeIndex + 1;
+
+    // Start a new epoch to discard all frame script messages relating to a
+    // previous epoch. All async messages that are still on their way to chrome
+    // will be ignored and don't override any tab data set when restoring.
+    let epoch = this.startNextEpoch(browser);
+
+    // keep the data around to prevent dataloss in case
+    // a tab gets closed before it's been properly restored
+    browser.__SS_restoreState = TAB_STATE_NEEDS_RESTORE;
+    browser.setAttribute("pending", "true");
+    tab.setAttribute("pending", "true");
+
+    // If we're restoring this tab, it certainly shouldn't be in
+    // the ignored set anymore.
+    this._crashedBrowsers.delete(browser.permanentKey);
+
+    // Update the persistent tab state cache with |tabData| information.
+    TabStateCache.update(browser, {
+      history: {entries: tabData.entries, index: tabData.index},
+      scroll: tabData.scroll || null,
+      storage: tabData.storage || null,
+      formdata: tabData.formdata || null,
+      disallow: tabData.disallow || null,
+      pageStyle: tabData.pageStyle || null,
+
+      // This information is only needed until the tab has finished restoring.
+      // When that's done it will be removed from the cache and we always
+      // collect it in TabState._collectBaseTabData().
+      image: tabData.image || "",
+      iconLoadingPrincipal: tabData.iconLoadingPrincipal || null,
+      userTypedValue: tabData.userTypedValue || "",
+      userTypedClear: tabData.userTypedClear || 0
+    });
+
+    browser.messageManager.sendAsyncMessage("SessionStore:restoreHistory",
+                                            {tabData: tabData, epoch: epoch, loadArguments});
+
+    // Restore tab attributes.
+    if ("attributes" in tabData) {
+      TabAttributes.set(tab, tabData.attributes);
+    }
+
+    // This could cause us to ignore MAX_CONCURRENT_TAB_RESTORES a bit, but
+    // it ensures each window will have its selected tab loaded.
+    if (willRestoreImmediately) {
+      this.restoreTabContent(tab, loadArguments, reloadInFreshProcess);
+    } else if (!forceOnDemand) {
+      this.restoreNextTab();
+    }
+
+    // Decrease the busy state counter after we're done.
+    this._setWindowStateReady(window);
+  },
+
+  /**
+   * Kicks off restoring the given tab.
+   *
+   * @param aTab
+   *        the tab to restore
+   * @param aLoadArguments
+   *        optional load arguments used for loadURI()
+   * @param aReloadInFreshProcess
+   *        true if we want to reload into a fresh process
+   */
+  restoreTabContent: function (aTab, aLoadArguments = null, aReloadInFreshProcess = false) {
+    if (aTab.hasAttribute("customizemode") && !aLoadArguments) {
+      return;
+    }
+
+    let browser = aTab.linkedBrowser;
+    let window = aTab.ownerGlobal;
+    let tabbrowser = window.gBrowser;
+    let tabData = TabState.clone(aTab);
+    let activeIndex = tabData.index - 1;
+    let activePageData = tabData.entries[activeIndex] || null;
+    let uri = activePageData ? activePageData.url || null : null;
+    if (aLoadArguments) {
+      uri = aLoadArguments.uri;
+      if (aLoadArguments.userContextId) {
+        browser.setAttribute("usercontextid", aLoadArguments.userContextId);
+      }
+    }
+
+    // We have to mark this tab as restoring first, otherwise
+    // the "pending" attribute will be applied to the linked
+    // browser, which removes it from the display list. We cannot
+    // flip the remoteness of any browser that is not being displayed.
+    this.markTabAsRestoring(aTab);
+
+    let isRemotenessUpdate = false;
+    if (aReloadInFreshProcess) {
+      isRemotenessUpdate = tabbrowser.switchBrowserIntoFreshProcess(browser);
+    } else {
+      isRemotenessUpdate = tabbrowser.updateBrowserRemotenessByURL(browser, uri);
+    }
+
+    if (isRemotenessUpdate) {
+      // We updated the remoteness, so we need to send the history down again.
+      //
+      // Start a new epoch to discard all frame script messages relating to a
+      // previous epoch. All async messages that are still on their way to chrome
+      // will be ignored and don't override any tab data set when restoring.
+      let epoch = this.startNextEpoch(browser);
+
+      browser.messageManager.sendAsyncMessage("SessionStore:restoreHistory", {
+        tabData: tabData,
+        epoch: epoch,
+        loadArguments: aLoadArguments,
+        isRemotenessUpdate,
+      });
+
+    }
+
+    // If the restored browser wants to show view source content, start up a
+    // view source browser that will load the required frame script.
+    if (uri && ViewSourceBrowser.isViewSource(uri)) {
+      new ViewSourceBrowser(browser);
+    }
+
+    browser.messageManager.sendAsyncMessage("SessionStore:restoreTabContent",
+      {loadArguments: aLoadArguments, isRemotenessUpdate});
+  },
+
+  /**
+   * Marks a given pending tab as restoring.
+   *
+   * @param aTab
+   *        the pending tab to mark as restoring
+   */
+  markTabAsRestoring(aTab) {
+    let browser = aTab.linkedBrowser;
+    if (browser.__SS_restoreState != TAB_STATE_NEEDS_RESTORE) {
+      throw new Error("Given tab is not pending.");
+    }
+
+    // Make sure that this tab is removed from the priority queue.
+    TabRestoreQueue.remove(aTab);
+
+    // Increase our internal count.
+    this._tabsRestoringCount++;
+
+    // Set this tab's state to restoring
+    browser.__SS_restoreState = TAB_STATE_RESTORING;
+    browser.removeAttribute("pending");
+    aTab.removeAttribute("pending");
+  },
+
+  /**
+   * This _attempts_ to restore the next available tab. If the restore fails,
+   * then we will attempt the next one.
+   * There are conditions where this won't do anything:
+   *   if we're in the process of quitting
+   *   if there are no tabs to restore
+   *   if we have already reached the limit for number of tabs to restore
+   */
+  restoreNextTab: function ssi_restoreNextTab() {
+    // If we call in here while quitting, we don't actually want to do anything
+    if (RunState.isQuitting)
+      return;
+
+    // Don't exceed the maximum number of concurrent tab restores.
+    if (this._tabsRestoringCount >= MAX_CONCURRENT_TAB_RESTORES)
+      return;
+
+    let tab = TabRestoreQueue.shift();
+    if (tab) {
+      this.restoreTabContent(tab);
+    }
+  },
+
+  /**
+   * Restore visibility and dimension features to a window
+   * @param aWindow
+   *        Window reference
+   * @param aWinData
+   *        Object containing session data for the window
+   */
+  restoreWindowFeatures: function ssi_restoreWindowFeatures(aWindow, aWinData) {
+    var hidden = (aWinData.hidden)?aWinData.hidden.split(","):[];
+    WINDOW_HIDEABLE_FEATURES.forEach(function(aItem) {
+      aWindow[aItem].visible = hidden.indexOf(aItem) == -1;
+    });
+
+    if (aWinData.isPopup) {
+      this._windows[aWindow.__SSi].isPopup = true;
+      if (aWindow.gURLBar) {
+        aWindow.gURLBar.readOnly = true;
+        aWindow.gURLBar.setAttribute("enablehistory", "false");
+      }
+    }
+    else {
+      delete this._windows[aWindow.__SSi].isPopup;
+      if (aWindow.gURLBar) {
+        aWindow.gURLBar.readOnly = false;
+        aWindow.gURLBar.setAttribute("enablehistory", "true");
+      }
+    }
+
+    var _this = this;
+    aWindow.setTimeout(function() {
+      _this.restoreDimensions.apply(_this, [aWindow,
+        +(aWinData.width || 0),
+        +(aWinData.height || 0),
+        "screenX" in aWinData ? +aWinData.screenX : NaN,
+        "screenY" in aWinData ? +aWinData.screenY : NaN,
+        aWinData.sizemode || "", aWinData.sidebar || ""]);
+    }, 0);
+  },
+
+  /**
+   * Restore a window's dimensions
+   * @param aWidth
+   *        Window width
+   * @param aHeight
+   *        Window height
+   * @param aLeft
+   *        Window left
+   * @param aTop
+   *        Window top
+   * @param aSizeMode
+   *        Window size mode (eg: maximized)
+   * @param aSidebar
+   *        Sidebar command
+   */
+  restoreDimensions: function ssi_restoreDimensions(aWindow, aWidth, aHeight, aLeft, aTop, aSizeMode, aSidebar) {
+    var win = aWindow;
+    var _this = this;
+    function win_(aName) { return _this._getWindowDimension(win, aName); }
+
+    // find available space on the screen where this window is being placed
+    let screen = gScreenManager.screenForRect(aLeft, aTop, aWidth, aHeight);
+    if (screen) {
+      let screenLeft = {}, screenTop = {}, screenWidth = {}, screenHeight = {};
+      screen.GetAvailRectDisplayPix(screenLeft, screenTop, screenWidth, screenHeight);
+      // screenX/Y are based on the origin of the screen's desktop-pixel coordinate space
+      let screenLeftCss = screenLeft.value;
+      let screenTopCss = screenTop.value;
+      // convert screen's device pixel dimensions to CSS px dimensions
+      screen.GetAvailRect(screenLeft, screenTop, screenWidth, screenHeight);
+      let cssToDevScale = screen.defaultCSSScaleFactor;
+      let screenRightCss = screenLeftCss + screenWidth.value / cssToDevScale;
+      let screenBottomCss = screenTopCss + screenHeight.value / cssToDevScale;
+
+      // Pull the window within the screen's bounds (allowing a little slop
+      // for windows that may be deliberately placed with their border off-screen
+      // as when Win10 "snaps" a window to the left/right edge -- bug 1276516).
+      // First, ensure the left edge is large enough...
+      if (aLeft < screenLeftCss - SCREEN_EDGE_SLOP) {
+        aLeft = screenLeftCss;
+      }
+      // Then check the resulting right edge, and reduce it if necessary.
+      let right = aLeft + aWidth;
+      if (right > screenRightCss + SCREEN_EDGE_SLOP) {
+        right = screenRightCss;
+        // See if we can move the left edge leftwards to maintain width.
+        if (aLeft > screenLeftCss) {
+          aLeft = Math.max(right - aWidth, screenLeftCss);
+        }
+      }
+      // Finally, update aWidth to account for the adjusted left and right edges.
+      aWidth = right - aLeft;
+
+      // And do the same in the vertical dimension.
+      if (aTop < screenTopCss - SCREEN_EDGE_SLOP) {
+        aTop = screenTopCss;
+      }
+      let bottom = aTop + aHeight;
+      if (bottom > screenBottomCss + SCREEN_EDGE_SLOP) {
+        bottom = screenBottomCss;
+        if (aTop > screenTopCss) {
+          aTop = Math.max(bottom - aHeight, screenTopCss);
+        }
+      }
+      aHeight = bottom - aTop;
+    }
+
+    // only modify those aspects which aren't correct yet
+    if (!isNaN(aLeft) && !isNaN(aTop) && (aLeft != win_("screenX") || aTop != win_("screenY"))) {
+      aWindow.moveTo(aLeft, aTop);
+    }
+    if (aWidth && aHeight && (aWidth != win_("width") || aHeight != win_("height"))) {
+      // Don't resize the window if it's currently maximized and we would
+      // maximize it again shortly after.
+      if (aSizeMode != "maximized" || win_("sizemode") != "maximized") {
+        aWindow.resizeTo(aWidth, aHeight);
+      }
+    }
+    if (aSizeMode && win_("sizemode") != aSizeMode)
+    {
+      switch (aSizeMode)
+      {
+      case "maximized":
+        aWindow.maximize();
+        break;
+      case "minimized":
+        aWindow.minimize();
+        break;
+      case "normal":
+        aWindow.restore();
+        break;
+      }
+    }
+    var sidebar = aWindow.document.getElementById("sidebar-box");
+    if (sidebar.getAttribute("sidebarcommand") != aSidebar) {
+      aWindow.SidebarUI.show(aSidebar);
+    }
+    // since resizing/moving a window brings it to the foreground,
+    // we might want to re-focus the last focused window
+    if (this.windowToFocus) {
+      this.windowToFocus.focus();
+    }
+  },
+
+  /* ........ Disk Access .............. */
+
+  /**
+   * Save the current session state to disk, after a delay.
+   *
+   * @param aWindow (optional)
+   *        Will mark the given window as dirty so that we will recollect its
+   *        data before we start writing.
+   */
+  saveStateDelayed: function (aWindow = null) {
+    if (aWindow) {
+      DirtyWindows.add(aWindow);
+    }
+
+    SessionSaver.runDelayed();
+  },
+
+  /* ........ Auxiliary Functions .............. */
+
+  /**
+   * Determines whether or not a tab that is being restored needs
+   * to have its remoteness flipped first.
+   *
+   * @param (object) with the following properties:
+   *
+   *        tabbrowser (<xul:tabbrowser>):
+   *          The tabbrowser that the browser belongs to.
+   *
+   *        tab (<xul:tab>):
+   *          The tab being restored
+   *
+   *        willRestoreImmediately (bool):
+   *          true if the tab is going to have its content
+   *          restored immediately by the caller.
+   *
+   */
+  _maybeUpdateBrowserRemoteness({ tabbrowser, tab,
+                                  willRestoreImmediately }) {
+    // If the browser we're attempting to restore happens to be
+    // remote, we need to flip it back to non-remote if it's going
+    // to go into the pending background tab state. This is to make
+    // sure that a background tab can't crash if it hasn't yet
+    // been restored.
+    //
+    // Normally, when a window is restored, the tabs that SessionStore
+    // inserts are non-remote - but the initial browser is, by default,
+    // remote, so this check and flip covers this case. The other case
+    // is when window state is overwriting the state of an existing
+    // window with some remote tabs.
+    let browser = tab.linkedBrowser;
+
+    // There are two ways that a tab might start restoring its content
+    // very soon - either the caller is going to restore the content
+    // immediately, or the TabRestoreQueue is set up so that the tab
+    // content is going to be restored in the very near future. In
+    // either case, we don't want to flip remoteness, since the browser
+    // will soon be loading content.
+    let willRestore = willRestoreImmediately ||
+                      TabRestoreQueue.willRestoreSoon(tab);
+
+    if (browser.isRemoteBrowser && !willRestore) {
+      tabbrowser.updateBrowserRemoteness(browser, false);
+    }
+  },
+
+  /**
+   * Update the session start time and send a telemetry measurement
+   * for the number of days elapsed since the session was started.
+   *
+   * @param state
+   *        The session state.
+   */
+  _updateSessionStartTime: function ssi_updateSessionStartTime(state) {
+    // Attempt to load the session start time from the session state
+    if (state.session && state.session.startTime) {
+      this._sessionStartTime = state.session.startTime;
+    }
+  },
+
+  /**
+   * call a callback for all currently opened browser windows
+   * (might miss the most recent one)
+   * @param aFunc
+   *        Callback each window is passed to
+   */
+  _forEachBrowserWindow: function ssi_forEachBrowserWindow(aFunc) {
+    var windowsEnum = Services.wm.getEnumerator("navigator:browser");
+
+    while (windowsEnum.hasMoreElements()) {
+      var window = windowsEnum.getNext();
+      if (window.__SSi && !window.closed) {
+        aFunc.call(this, window);
+      }
+    }
+  },
+
+  /**
+   * Returns most recent window
+   * @returns Window reference
+   */
+  _getMostRecentBrowserWindow: function ssi_getMostRecentBrowserWindow() {
+    return RecentWindow.getMostRecentBrowserWindow({ allowPopups: true });
+  },
+
+  /**
+   * Calls onClose for windows that are determined to be closed but aren't
+   * destroyed yet, which would otherwise cause getBrowserState and
+   * setBrowserState to treat them as open windows.
+   */
+  _handleClosedWindows: function ssi_handleClosedWindows() {
+    var windowsEnum = Services.wm.getEnumerator("navigator:browser");
+
+    while (windowsEnum.hasMoreElements()) {
+      var window = windowsEnum.getNext();
+      if (window.closed) {
+        this.onClose(window);
+      }
+    }
+  },
+
+  /**
+   * open a new browser window for a given session state
+   * called when restoring a multi-window session
+   * @param aState
+   *        Object containing session data
+   */
+  _openWindowWithState: function ssi_openWindowWithState(aState) {
+    var argString = Cc["@mozilla.org/supports-string;1"].
+                    createInstance(Ci.nsISupportsString);
+    argString.data = "";
+
+    // Build feature string
+    let features = "chrome,dialog=no,macsuppressanimation,all";
+    let winState = aState.windows[0];
+    WINDOW_ATTRIBUTES.forEach(function(aFeature) {
+      // Use !isNaN as an easy way to ignore sizemode and check for numbers
+      if (aFeature in winState && !isNaN(winState[aFeature]))
+        features += "," + aFeature + "=" + winState[aFeature];
+    });
+
+    if (winState.isPrivate) {
+      features += ",private";
+    }
+
+    var window =
+      Services.ww.openWindow(null, this._prefBranch.getCharPref("chromeURL"),
+                             "_blank", features, argString);
+
+    do {
+      var ID = "window" + Math.random();
+    } while (ID in this._statesToRestore);
+    this._statesToRestore[(window.__SS_restoreID = ID)] = aState;
+
+    return window;
+  },
+
+  /**
+   * Whether or not to resume session, if not recovering from a crash.
+   * @returns bool
+   */
+  _doResumeSession: function ssi_doResumeSession() {
+    return this._prefBranch.getIntPref("startup.page") == 3 ||
+           this._prefBranch.getBoolPref("sessionstore.resume_session_once");
+  },
+
+  /**
+   * whether the user wants to load any other page at startup
+   * (except the homepage) - needed for determining whether to overwrite the current tabs
+   * C.f.: nsBrowserContentHandler's defaultArgs implementation.
+   * @returns bool
+   */
+  _isCmdLineEmpty: function ssi_isCmdLineEmpty(aWindow, aState) {
+    var pinnedOnly = aState.windows &&
+                     aState.windows.every(win =>
+                       win.tabs.every(tab => tab.pinned));
+
+    let hasFirstArgument = aWindow.arguments && aWindow.arguments[0];
+    if (!pinnedOnly) {
+      let defaultArgs = Cc["@mozilla.org/browser/clh;1"].
+                        getService(Ci.nsIBrowserHandler).defaultArgs;
+      if (aWindow.arguments &&
+          aWindow.arguments[0] &&
+          aWindow.arguments[0] == defaultArgs)
+        hasFirstArgument = false;
+    }
+
+    return !hasFirstArgument;
+  },
+
+  /**
+   * on popup windows, the XULWindow's attributes seem not to be set correctly
+   * we use thus JSDOMWindow attributes for sizemode and normal window attributes
+   * (and hope for reasonable values when maximized/minimized - since then
+   * outerWidth/outerHeight aren't the dimensions of the restored window)
+   * @param aWindow
+   *        Window reference
+   * @param aAttribute
+   *        String sizemode | width | height | other window attribute
+   * @returns string
+   */
+  _getWindowDimension: function ssi_getWindowDimension(aWindow, aAttribute) {
+    if (aAttribute == "sizemode") {
+      switch (aWindow.windowState) {
+      case aWindow.STATE_FULLSCREEN:
+      case aWindow.STATE_MAXIMIZED:
+        return "maximized";
+      case aWindow.STATE_MINIMIZED:
+        return "minimized";
+      default:
+        return "normal";
+      }
+    }
+
+    var dimension;
+    switch (aAttribute) {
+    case "width":
+      dimension = aWindow.outerWidth;
+      break;
+    case "height":
+      dimension = aWindow.outerHeight;
+      break;
+    default:
+      dimension = aAttribute in aWindow ? aWindow[aAttribute] : "";
+      break;
+    }
+
+    if (aWindow.windowState == aWindow.STATE_NORMAL) {
+      return dimension;
+    }
+    return aWindow.document.documentElement.getAttribute(aAttribute) || dimension;
+  },
+
+  /**
+   * @param aState is a session state
+   * @param aRecentCrashes is the number of consecutive crashes
+   * @returns whether a restore page will be needed for the session state
+   */
+  _needsRestorePage: function ssi_needsRestorePage(aState, aRecentCrashes) {
+    const SIX_HOURS_IN_MS = 6 * 60 * 60 * 1000;
+
+    // don't display the page when there's nothing to restore
+    let winData = aState.windows || null;
+    if (!winData || winData.length == 0)
+      return false;
+
+    // don't wrap a single about:sessionrestore page
+    if (this._hasSingleTabWithURL(winData, "about:sessionrestore") ||
+        this._hasSingleTabWithURL(winData, "about:welcomeback")) {
+      return false;
+    }
+
+    // don't automatically restore in Safe Mode
+    if (Services.appinfo.inSafeMode)
+      return true;
+
+    let max_resumed_crashes =
+      this._prefBranch.getIntPref("sessionstore.max_resumed_crashes");
+    let sessionAge = aState.session && aState.session.lastUpdate &&
+                     (Date.now() - aState.session.lastUpdate);
+
+    return max_resumed_crashes != -1 &&
+           (aRecentCrashes > max_resumed_crashes ||
+            sessionAge && sessionAge >= SIX_HOURS_IN_MS);
+  },
+
+  /**
+   * @param aWinData is the set of windows in session state
+   * @param aURL is the single URL we're looking for
+   * @returns whether the window data contains only the single URL passed
+   */
+  _hasSingleTabWithURL: function(aWinData, aURL) {
+    if (aWinData &&
+        aWinData.length == 1 &&
+        aWinData[0].tabs &&
+        aWinData[0].tabs.length == 1 &&
+        aWinData[0].tabs[0].entries &&
+        aWinData[0].tabs[0].entries.length == 1) {
+      return aURL == aWinData[0].tabs[0].entries[0].url;
+    }
+    return false;
+  },
+
+  /**
+   * Determine if the tab state we're passed is something we should save. This
+   * is used when closing a tab or closing a window with a single tab
+   *
+   * @param aTabState
+   *        The current tab state
+   * @returns boolean
+   */
+  _shouldSaveTabState: function ssi_shouldSaveTabState(aTabState) {
+    // If the tab has only a transient about: history entry, no other
+    // session history, and no userTypedValue, then we don't actually want to
+    // store this tab's data.
+    return aTabState.entries.length &&
+           !(aTabState.entries.length == 1 &&
+                (aTabState.entries[0].url == "about:blank" ||
+                 aTabState.entries[0].url == "about:newtab" ||
+                 aTabState.entries[0].url == "about:privatebrowsing") &&
+                 !aTabState.userTypedValue);
+  },
+
+  /**
+   * This is going to take a state as provided at startup (via
+   * nsISessionStartup.state) and split it into 2 parts. The first part
+   * (defaultState) will be a state that should still be restored at startup,
+   * while the second part (state) is a state that should be saved for later.
+   * defaultState will be comprised of windows with only pinned tabs, extracted
+   * from state. It will contain the cookies that go along with the history
+   * entries in those tabs. It will also contain window position information.
+   *
+   * defaultState will be restored at startup. state will be passed into
+   * LastSession and will be kept in case the user explicitly wants
+   * to restore the previous session (publicly exposed as restoreLastSession).
+   *
+   * @param state
+   *        The state, presumably from nsISessionStartup.state
+   * @returns [defaultState, state]
+   */
+  _prepDataForDeferredRestore: function ssi_prepDataForDeferredRestore(state) {
+    // Make sure that we don't modify the global state as provided by
+    // nsSessionStartup.state.
+    state = Cu.cloneInto(state, {});
+
+    let defaultState = { windows: [], selectedWindow: 1 };
+
+    state.selectedWindow = state.selectedWindow || 1;
+
+    // Look at each window, remove pinned tabs, adjust selectedindex,
+    // remove window if necessary.
+    for (let wIndex = 0; wIndex < state.windows.length;) {
+      let window = state.windows[wIndex];
+      window.selected = window.selected || 1;
+      // We're going to put the state of the window into this object
+      let pinnedWindowState = { tabs: [], cookies: []};
+      for (let tIndex = 0; tIndex < window.tabs.length;) {
+        if (window.tabs[tIndex].pinned) {
+          // Adjust window.selected
+          if (tIndex + 1 < window.selected)
+            window.selected -= 1;
+          else if (tIndex + 1 == window.selected)
+            pinnedWindowState.selected = pinnedWindowState.tabs.length + 2;
+            // + 2 because the tab isn't actually in the array yet
+
+          // Now add the pinned tab to our window
+          pinnedWindowState.tabs =
+            pinnedWindowState.tabs.concat(window.tabs.splice(tIndex, 1));
+          // We don't want to increment tIndex here.
+          continue;
+        }
+        tIndex++;
+      }
+
+      // At this point the window in the state object has been modified (or not)
+      // We want to build the rest of this new window object if we have pinnedTabs.
+      if (pinnedWindowState.tabs.length) {
+        // First get the other attributes off the window
+        WINDOW_ATTRIBUTES.forEach(function(attr) {
+          if (attr in window) {
+            pinnedWindowState[attr] = window[attr];
+            delete window[attr];
+          }
+        });
+        // We're just copying position data into the pinned window.
+        // Not copying over:
+        // - _closedTabs
+        // - extData
+        // - isPopup
+        // - hidden
+
+        // Assign a unique ID to correlate the window to be opened with the
+        // remaining data
+        window.__lastSessionWindowID = pinnedWindowState.__lastSessionWindowID
+                                     = "" + Date.now() + Math.random();
+
+        // Extract the cookies that belong with each pinned tab
+        this._splitCookiesFromWindow(window, pinnedWindowState);
+
+        // Actually add this window to our defaultState
+        defaultState.windows.push(pinnedWindowState);
+        // Remove the window from the state if it doesn't have any tabs
+        if (!window.tabs.length) {
+          if (wIndex + 1 <= state.selectedWindow)
+            state.selectedWindow -= 1;
+          else if (wIndex + 1 == state.selectedWindow)
+            defaultState.selectedIndex = defaultState.windows.length + 1;
+
+          state.windows.splice(wIndex, 1);
+          // We don't want to increment wIndex here.
+          continue;
+        }
+
+
+      }
+      wIndex++;
+    }
+
+    return [defaultState, state];
+  },
+
+  /**
+   * Splits out the cookies from aWinState into aTargetWinState based on the
+   * tabs that are in aTargetWinState.
+   * This alters the state of aWinState and aTargetWinState.
+   */
+  _splitCookiesFromWindow:
+    function ssi_splitCookiesFromWindow(aWinState, aTargetWinState) {
+    if (!aWinState.cookies || !aWinState.cookies.length)
+      return;
+
+    // Get the hosts for history entries in aTargetWinState
+    let cookieHosts = SessionCookies.getHostsForWindow(aTargetWinState);
+
+    // By creating a regex we reduce overhead and there is only one loop pass
+    // through either array (cookieHosts and aWinState.cookies).
+    let hosts = Object.keys(cookieHosts).join("|").replace(/\./g, "\\.");
+    // If we don't actually have any hosts, then we don't want to do anything.
+    if (!hosts.length)
+      return;
+    let cookieRegex = new RegExp(".*(" + hosts + ")");
+    for (let cIndex = 0; cIndex < aWinState.cookies.length;) {
+      if (cookieRegex.test(aWinState.cookies[cIndex].host)) {
+        aTargetWinState.cookies =
+          aTargetWinState.cookies.concat(aWinState.cookies.splice(cIndex, 1));
+        continue;
+      }
+      cIndex++;
+    }
+  },
+
+  _sendRestoreCompletedNotifications: function ssi_sendRestoreCompletedNotifications() {
+    // not all windows restored, yet
+    if (this._restoreCount > 1) {
+      this._restoreCount--;
+      return;
+    }
+
+    // observers were already notified
+    if (this._restoreCount == -1)
+      return;
+
+    // This was the last window restored at startup, notify observers.
+    Services.obs.notifyObservers(null,
+      this._browserSetState ? NOTIFY_BROWSER_STATE_RESTORED : NOTIFY_WINDOWS_RESTORED,
+      "");
+
+    this._browserSetState = false;
+    this._restoreCount = -1;
+  },
+
+   /**
+   * Set the given window's busy state
+   * @param aWindow the window
+   * @param aValue the window's busy state
+   */
+  _setWindowStateBusyValue:
+    function ssi_changeWindowStateBusyValue(aWindow, aValue) {
+
+    this._windows[aWindow.__SSi].busy = aValue;
+
+    // Keep the to-be-restored state in sync because that is returned by
+    // getWindowState() as long as the window isn't loaded, yet.
+    if (!this._isWindowLoaded(aWindow)) {
+      let stateToRestore = this._statesToRestore[aWindow.__SS_restoreID].windows[0];
+      stateToRestore.busy = aValue;
+    }
+  },
+
+  /**
+   * Set the given window's state to 'not busy'.
+   * @param aWindow the window
+   */
+  _setWindowStateReady: function ssi_setWindowStateReady(aWindow) {
+    let newCount = (this._windowBusyStates.get(aWindow) || 0) - 1;
+    if (newCount < 0) {
+      throw new Error("Invalid window busy state (less than zero).");
+    }
+    this._windowBusyStates.set(aWindow, newCount);
+
+    if (newCount == 0) {
+      this._setWindowStateBusyValue(aWindow, false);
+      this._sendWindowStateEvent(aWindow, "Ready");
+    }
+  },
+
+  /**
+   * Set the given window's state to 'busy'.
+   * @param aWindow the window
+   */
+  _setWindowStateBusy: function ssi_setWindowStateBusy(aWindow) {
+    let newCount = (this._windowBusyStates.get(aWindow) || 0) + 1;
+    this._windowBusyStates.set(aWindow, newCount);
+
+    if (newCount == 1) {
+      this._setWindowStateBusyValue(aWindow, true);
+      this._sendWindowStateEvent(aWindow, "Busy");
+    }
+  },
+
+  /**
+   * Dispatch an SSWindowState_____ event for the given window.
+   * @param aWindow the window
+   * @param aType the type of event, SSWindowState will be prepended to this string
+   */
+  _sendWindowStateEvent: function ssi_sendWindowStateEvent(aWindow, aType) {
+    let event = aWindow.document.createEvent("Events");
+    event.initEvent("SSWindowState" + aType, true, false);
+    aWindow.dispatchEvent(event);
+  },
+
+  /**
+   * Dispatch the SSWindowRestored event for the given window.
+   * @param aWindow
+   *        The window which has been restored
+   */
+  _sendWindowRestoredNotification(aWindow) {
+    let event = aWindow.document.createEvent("Events");
+    event.initEvent("SSWindowRestored", true, false);
+    aWindow.dispatchEvent(event);
+  },
+
+  /**
+   * Dispatch the SSTabRestored event for the given tab.
+   * @param aTab
+   *        The tab which has been restored
+   * @param aIsRemotenessUpdate
+   *        True if this tab was restored due to flip from running from
+   *        out-of-main-process to in-main-process or vice-versa.
+   */
+  _sendTabRestoredNotification(aTab, aIsRemotenessUpdate) {
+    let event = aTab.ownerDocument.createEvent("CustomEvent");
+    event.initCustomEvent("SSTabRestored", true, false, {
+      isRemotenessUpdate: aIsRemotenessUpdate,
+    });
+    aTab.dispatchEvent(event);
+  },
+
+  /**
+   * @param aWindow
+   *        Window reference
+   * @returns whether this window's data is still cached in _statesToRestore
+   *          because it's not fully loaded yet
+   */
+  _isWindowLoaded: function ssi_isWindowLoaded(aWindow) {
+    return !aWindow.__SS_restoreID;
+  },
+
+  /**
+   * Replace "Loading..." with the tab label (with minimal side-effects)
+   * @param aString is the string the title is stored in
+   * @param aTabbrowser is a tabbrowser object, containing aTab
+   * @param aTab is the tab whose title we're updating & using
+   *
+   * @returns aString that has been updated with the new title
+   */
+  _replaceLoadingTitle : function ssi_replaceLoadingTitle(aString, aTabbrowser, aTab) {
+    if (aString == aTabbrowser.mStringBundle.getString("tabs.connecting")) {
+      aTabbrowser.setTabTitle(aTab);
+      [aString, aTab.label] = [aTab.label, aString];
+    }
+    return aString;
+  },
+
+  /**
+   * Resize this._closedWindows to the value of the pref, except in the case
+   * where we don't have any non-popup windows on Windows and Linux. Then we must
+   * resize such that we have at least one non-popup window.
+   */
+  _capClosedWindows : function ssi_capClosedWindows() {
+    if (this._closedWindows.length <= this._max_windows_undo)
+      return;
+    let spliceTo = this._max_windows_undo;
+    if (AppConstants.platform != "macosx") {
+      let normalWindowIndex = 0;
+      // try to find a non-popup window in this._closedWindows
+      while (normalWindowIndex < this._closedWindows.length &&
+             !!this._closedWindows[normalWindowIndex].isPopup)
+        normalWindowIndex++;
+      if (normalWindowIndex >= this._max_windows_undo)
+        spliceTo = normalWindowIndex + 1;
+    }
+    this._closedWindows.splice(spliceTo, this._closedWindows.length);
+  },
+
+  /**
+   * Clears the set of windows that are "resurrected" before writing to disk to
+   * make closing windows one after the other until shutdown work as expected.
+   *
+   * This function should only be called when we are sure that there has been
+   * a user action that indicates the browser is actively being used and all
+   * windows that have been closed before are not part of a series of closing
+   * windows.
+   */
+  _clearRestoringWindows: function ssi_clearRestoringWindows() {
+    for (let i = 0; i < this._closedWindows.length; i++) {
+      delete this._closedWindows[i]._shouldRestore;
+    }
+  },
+
+  /**
+   * Reset state to prepare for a new session state to be restored.
+   */
+  _resetRestoringState: function ssi_initRestoringState() {
+    TabRestoreQueue.reset();
+    this._tabsRestoringCount = 0;
+  },
+
+  /**
+   * Reset the restoring state for a particular tab. This will be called when
+   * removing a tab or when a tab needs to be reset (it's being overwritten).
+   *
+   * @param aTab
+   *        The tab that will be "reset"
+   */
+  _resetLocalTabRestoringState: function (aTab) {
+    NS_ASSERT(aTab.linkedBrowser.__SS_restoreState,
+              "given tab is not restoring");
+
+    let browser = aTab.linkedBrowser;
+
+    // Keep the tab's previous state for later in this method
+    let previousState = browser.__SS_restoreState;
+
+    // The browser is no longer in any sort of restoring state.
+    delete browser.__SS_restoreState;
+
+    aTab.removeAttribute("pending");
+    browser.removeAttribute("pending");
+
+    if (previousState == TAB_STATE_RESTORING) {
+      if (this._tabsRestoringCount)
+        this._tabsRestoringCount--;
+    } else if (previousState == TAB_STATE_NEEDS_RESTORE) {
+      // Make sure that the tab is removed from the list of tabs to restore.
+      // Again, this is normally done in restoreTabContent, but that isn't being called
+      // for this tab.
+      TabRestoreQueue.remove(aTab);
+    }
+  },
+
+  _resetTabRestoringState: function (tab) {
+    NS_ASSERT(tab.linkedBrowser.__SS_restoreState,
+              "given tab is not restoring");
+
+    let browser = tab.linkedBrowser;
+    browser.messageManager.sendAsyncMessage("SessionStore:resetRestore", {});
+    this._resetLocalTabRestoringState(tab);
+  },
+
+  /**
+   * Each fresh tab starts out with epoch=0. This function can be used to
+   * start a next epoch by incrementing the current value. It will enables us
+   * to ignore stale messages sent from previous epochs. The function returns
+   * the new epoch ID for the given |browser|.
+   */
+  startNextEpoch(browser) {
+    let next = this.getCurrentEpoch(browser) + 1;
+    this._browserEpochs.set(browser.permanentKey, next);
+    return next;
+  },
+
+  /**
+   * Returns the current epoch for the given <browser>. If we haven't assigned
+   * a new epoch this will default to zero for new tabs.
+   */
+  getCurrentEpoch(browser) {
+    return this._browserEpochs.get(browser.permanentKey) || 0;
+  },
+
+  /**
+   * Each time a <browser> element is restored, we increment its "epoch". To
+   * check if a message from content-sessionStore.js is out of date, we can
+   * compare the epoch received with the message to the <browser> element's
+   * epoch. This function does that, and returns true if |epoch| is up-to-date
+   * with respect to |browser|.
+   */
+  isCurrentEpoch: function (browser, epoch) {
+    return this.getCurrentEpoch(browser) == epoch;
+  },
+
+  /**
+   * Resets the epoch for a given <browser>. We need to this every time we
+   * receive a hint that a new docShell has been loaded into the browser as
+   * the frame script starts out with epoch=0.
+   */
+  resetEpoch(browser) {
+    this._browserEpochs.delete(browser.permanentKey);
+  },
+
+  /**
+   * Handle an error report from a content process.
+   */
+  reportInternalError(data) {
+    // For the moment, we only report errors through Telemetry.
+    if (data.telemetry) {
+      for (let key of Object.keys(data.telemetry)) {
+        let histogram = Telemetry.getHistogramById(key);
+        histogram.add(data.telemetry[key]);
+      }
+    }
+  },
+
+  /**
+   * Countdown for a given duration, skipping beats if the computer is too busy,
+   * sleeping or otherwise unavailable.
+   *
+   * @param {number} delay An approximate delay to wait in milliseconds (rounded
+   * up to the closest second).
+   *
+   * @return Promise
+   */
+  looseTimer(delay) {
+    let DELAY_BEAT = 1000;
+    let timer = Cc["@mozilla.org/timer;1"].createInstance(Ci.nsITimer);
+    let beats = Math.ceil(delay / DELAY_BEAT);
+    let promise =  new Promise(resolve => {
+      timer.initWithCallback(function() {
+        if (beats <= 0) {
+          resolve();
+        }
+        --beats;
+      }, DELAY_BEAT, Ci.nsITimer.TYPE_REPEATING_PRECISE_CAN_SKIP);
+    });
+    // Ensure that the timer is both canceled once we are done with it
+    // and not garbage-collected until then.
+    promise.then(() => timer.cancel(), () => timer.cancel());
+    return promise;
+  }
+};
+
+/**
+ * Priority queue that keeps track of a list of tabs to restore and returns
+ * the tab we should restore next, based on priority rules. We decide between
+ * pinned, visible and hidden tabs in that and FIFO order. Hidden tabs are only
+ * restored with restore_hidden_tabs=true.
+ */
+var TabRestoreQueue = {
+  // The separate buckets used to store tabs.
+  tabs: {priority: [], visible: [], hidden: []},
+
+  // Preferences used by the TabRestoreQueue to determine which tabs
+  // are restored automatically and which tabs will be on-demand.
+  prefs: {
+    // Lazy getter that returns whether tabs are restored on demand.
+    get restoreOnDemand() {
+      let updateValue = () => {
+        let value = Services.prefs.getBoolPref(PREF);
+        let definition = {value: value, configurable: true};
+        Object.defineProperty(this, "restoreOnDemand", definition);
+        return value;
+      }
+
+      const PREF = "browser.sessionstore.restore_on_demand";
+      Services.prefs.addObserver(PREF, updateValue, false);
+      return updateValue();
+    },
+
+    // Lazy getter that returns whether pinned tabs are restored on demand.
+    get restorePinnedTabsOnDemand() {
+      let updateValue = () => {
+        let value = Services.prefs.getBoolPref(PREF);
+        let definition = {value: value, configurable: true};
+        Object.defineProperty(this, "restorePinnedTabsOnDemand", definition);
+        return value;
+      }
+
+      const PREF = "browser.sessionstore.restore_pinned_tabs_on_demand";
+      Services.prefs.addObserver(PREF, updateValue, false);
+      return updateValue();
+    },
+
+    // Lazy getter that returns whether we should restore hidden tabs.
+    get restoreHiddenTabs() {
+      let updateValue = () => {
+        let value = Services.prefs.getBoolPref(PREF);
+        let definition = {value: value, configurable: true};
+        Object.defineProperty(this, "restoreHiddenTabs", definition);
+        return value;
+      }
+
+      const PREF = "browser.sessionstore.restore_hidden_tabs";
+      Services.prefs.addObserver(PREF, updateValue, false);
+      return updateValue();
+    }
+  },
+
+  // Resets the queue and removes all tabs.
+  reset: function () {
+    this.tabs = {priority: [], visible: [], hidden: []};
+  },
+
+  // Adds a tab to the queue and determines its priority bucket.
+  add: function (tab) {
+    let {priority, hidden, visible} = this.tabs;
+
+    if (tab.pinned) {
+      priority.push(tab);
+    } else if (tab.hidden) {
+      hidden.push(tab);
+    } else {
+      visible.push(tab);
+    }
+  },
+
+  // Removes a given tab from the queue, if it's in there.
+  remove: function (tab) {
+    let {priority, hidden, visible} = this.tabs;
+
+    // We'll always check priority first since we don't
+    // have an indicator if a tab will be there or not.
+    let set = priority;
+    let index = set.indexOf(tab);
+
+    if (index == -1) {
+      set = tab.hidden ? hidden : visible;
+      index = set.indexOf(tab);
+    }
+
+    if (index > -1) {
+      set.splice(index, 1);
+    }
+  },
+
+  // Returns and removes the tab with the highest priority.
+  shift: function () {
+    let set;
+    let {priority, hidden, visible} = this.tabs;
+
+    let {restoreOnDemand, restorePinnedTabsOnDemand} = this.prefs;
+    let restorePinned = !(restoreOnDemand && restorePinnedTabsOnDemand);
+    if (restorePinned && priority.length) {
+      set = priority;
+    } else if (!restoreOnDemand) {
+      if (visible.length) {
+        set = visible;
+      } else if (this.prefs.restoreHiddenTabs && hidden.length) {
+        set = hidden;
+      }
+    }
+
+    return set && set.shift();
+  },
+
+  // Moves a given tab from the 'hidden' to the 'visible' bucket.
+  hiddenToVisible: function (tab) {
+    let {hidden, visible} = this.tabs;
+    let index = hidden.indexOf(tab);
+
+    if (index > -1) {
+      hidden.splice(index, 1);
+      visible.push(tab);
+    }
+  },
+
+  // Moves a given tab from the 'visible' to the 'hidden' bucket.
+  visibleToHidden: function (tab) {
+    let {visible, hidden} = this.tabs;
+    let index = visible.indexOf(tab);
+
+    if (index > -1) {
+      visible.splice(index, 1);
+      hidden.push(tab);
+    }
+  },
+
+  /**
+   * Returns true if the passed tab is in one of the sets that we're
+   * restoring content in automatically.
+   *
+   * @param tab (<xul:tab>)
+   *        The tab to check
+   * @returns bool
+   */
+  willRestoreSoon: function (tab) {
+    let { priority, hidden, visible } = this.tabs;
+    let { restoreOnDemand, restorePinnedTabsOnDemand,
+          restoreHiddenTabs } = this.prefs;
+    let restorePinned = !(restoreOnDemand && restorePinnedTabsOnDemand);
+    let candidateSet = [];
+
+    if (restorePinned && priority.length)
+      candidateSet.push(...priority);
+
+    if (!restoreOnDemand) {
+      if (visible.length)
+        candidateSet.push(...visible);
+
+      if (restoreHiddenTabs && hidden.length)
+        candidateSet.push(...hidden);
+    }
+
+    return candidateSet.indexOf(tab) > -1;
+  },
+};
+
+// A map storing a closed window's state data until it goes aways (is GC'ed).
+// This ensures that API clients can still read (but not write) states of
+// windows they still hold a reference to but we don't.
+var DyingWindowCache = {
+  _data: new WeakMap(),
+
+  has: function (window) {
+    return this._data.has(window);
+  },
+
+  get: function (window) {
+    return this._data.get(window);
+  },
+
+  set: function (window, data) {
+    this._data.set(window, data);
+  },
+
+  remove: function (window) {
+    this._data.delete(window);
+  }
+};
+
+// A weak set of dirty windows. We use it to determine which windows we need to
+// recollect data for when getCurrentState() is called.
+var DirtyWindows = {
+  _data: new WeakMap(),
+
+  has: function (window) {
+    return this._data.has(window);
+  },
+
+  add: function (window) {
+    return this._data.set(window, true);
+  },
+
+  remove: function (window) {
+    this._data.delete(window);
+  },
+
+  clear: function (window) {
+    this._data = new WeakMap();
+  }
+};
+
+// The state from the previous session (after restoring pinned tabs). This
+// state is persisted and passed through to the next session during an app
+// restart to make the third party add-on warning not trash the deferred
+// session
+var LastSession = {
+  _state: null,
+
+  get canRestore() {
+    return !!this._state;
+  },
+
+  getState: function () {
+    return this._state;
+  },
+
+  setState: function (state) {
+    this._state = state;
+  },
+
+  clear: function () {
+    if (this._state) {
+      this._state = null;
+      Services.obs.notifyObservers(null, NOTIFY_LAST_SESSION_CLEARED, null);
+    }
+  }
+};
diff --git a/application/basilisk/components/sessionstore/SessionWorker.js b/application/basilisk/components/sessionstore/SessionWorker.js
new file mode 100644
index 0000000000..7d802a7df8
--- /dev/null
+++ b/application/basilisk/components/sessionstore/SessionWorker.js
@@ -0,0 +1,381 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this file,
+ * You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+/**
+ * A worker dedicated to handle I/O for Session Store.
+ */
+
+"use strict";
+
+importScripts("resource://gre/modules/osfile.jsm");
+
+var PromiseWorker = require("resource://gre/modules/workers/PromiseWorker.js");
+
+var File = OS.File;
+var Encoder = new TextEncoder();
+var Decoder = new TextDecoder();
+
+var worker = new PromiseWorker.AbstractWorker();
+worker.dispatch = function(method, args = []) {
+  return Agent[method](...args);
+};
+worker.postMessage = function(result, ...transfers) {
+  self.postMessage(result, ...transfers);
+};
+worker.close = function() {
+  self.close();
+};
+
+self.addEventListener("message", msg => worker.handleMessage(msg));
+
+// The various possible states
+
+/**
+ * We just started (we haven't written anything to disk yet) from
+ * `Paths.clean`. The backup directory may not exist.
+ */
+const STATE_CLEAN = "clean";
+/**
+ * We know that `Paths.recovery` is good, either because we just read
+ * it (we haven't written anything to disk yet) or because have
+ * already written once to `Paths.recovery` during this session.
+ * `Paths.clean` is absent or invalid. The backup directory exists.
+ */
+const STATE_RECOVERY = "recovery";
+/**
+ * We just started from `Paths.recoverBackupy` (we haven't written
+ * anything to disk yet). Both `Paths.clean` and `Paths.recovery` are
+ * absent or invalid. The backup directory exists.
+ */
+const STATE_RECOVERY_BACKUP = "recoveryBackup";
+/**
+ * We just started from `Paths.upgradeBackup` (we haven't written
+ * anything to disk yet). Both `Paths.clean`, `Paths.recovery` and
+ * `Paths.recoveryBackup` are absent or invalid. The backup directory
+ * exists.
+ */
+const STATE_UPGRADE_BACKUP = "upgradeBackup";
+/**
+ * We just started without a valid session store file (we haven't
+ * written anything to disk yet). The backup directory may not exist.
+ */
+const STATE_EMPTY = "empty";
+
+var Agent = {
+  // Path to the files used by the SessionWorker
+  Paths: null,
+
+  /**
+   * The current state of the worker, as one of the following strings:
+   * - "permanent", once the first write has been completed;
+   * - "empty", before the first write has been completed,
+   *   if we have started without any sessionstore;
+   * - one of "clean", "recovery", "recoveryBackup", "cleanBackup",
+   *   "upgradeBackup", before the first write has been completed, if
+   *   we have started by loading the corresponding file.
+   */
+  state: null,
+
+  /**
+   * Number of old upgrade backups that are being kept
+   */
+  maxUpgradeBackups: null,
+
+  /**
+   * Initialize (or reinitialize) the worker
+   *
+   * @param {string} origin Which of sessionstore.js or its backups
+   *   was used. One of the `STATE_*` constants defined above.
+   * @param {object} paths The paths at which to find the various files.
+   * @param {object} prefs The preferences the worker needs to known.
+   */
+  init(origin, paths, prefs = {}) {
+    if (!(origin in paths || origin == STATE_EMPTY)) {
+      throw new TypeError("Invalid origin: " + origin);
+    }
+
+    // Check that all required preference values were passed.
+    for (let pref of ["maxUpgradeBackups", "maxSerializeBack", "maxSerializeForward"]) {
+      if (!prefs.hasOwnProperty(pref)) {
+        throw new TypeError(`Missing preference value for ${pref}`);
+      }
+    }
+
+    this.state = origin;
+    this.Paths = paths;
+    this.maxUpgradeBackups = prefs.maxUpgradeBackups;
+    this.maxSerializeBack = prefs.maxSerializeBack;
+    this.maxSerializeForward = prefs.maxSerializeForward;
+    this.upgradeBackupNeeded = paths.nextUpgradeBackup != paths.upgradeBackup;
+    return {result: true};
+  },
+
+  /**
+   * Write the session to disk.
+   * Write the session to disk, performing any necessary backup
+   * along the way.
+   *
+   * @param {object} state The state to write to disk.
+   * @param {object} options
+   *  - performShutdownCleanup If |true|, we should
+   *    perform shutdown-time cleanup to ensure that private data
+   *    is not left lying around;
+   *  - isFinalWrite If |true|, write to Paths.clean instead of
+   *    Paths.recovery
+   */
+  write: function (state, options = {}) {
+    let exn;
+    let telemetry = {};
+
+    // Cap the number of backward and forward shistory entries on shutdown.
+    if (options.isFinalWrite) {
+      for (let window of state.windows) {
+        for (let tab of window.tabs) {
+          let lower = 0;
+          let upper = tab.entries.length;
+
+          if (this.maxSerializeBack > -1) {
+            lower = Math.max(lower, tab.index - this.maxSerializeBack - 1);
+          }
+          if (this.maxSerializeForward > -1) {
+            upper = Math.min(upper, tab.index + this.maxSerializeForward);
+          }
+
+          tab.entries = tab.entries.slice(lower, upper);
+          tab.index -= lower;
+        }
+      }
+    }
+
+    let stateString = JSON.stringify(state);
+    let data = Encoder.encode(stateString);
+
+    try {
+
+      if (this.state == STATE_CLEAN || this.state == STATE_EMPTY) {
+        // The backups directory may not exist yet. In all other cases,
+        // we have either already read from or already written to this
+        // directory, so we are satisfied that it exists.
+        File.makeDir(this.Paths.backups);
+      }
+
+      if (this.state == STATE_CLEAN) {
+        // Move $Path.clean out of the way, to avoid any ambiguity as
+        // to which file is more recent.
+        File.move(this.Paths.clean, this.Paths.cleanBackup);
+      }
+
+      let startWriteMs = Date.now();
+
+      if (options.isFinalWrite) {
+        // We are shutting down. At this stage, we know that
+        // $Paths.clean is either absent or corrupted. If it was
+        // originally present and valid, it has been moved to
+        // $Paths.cleanBackup a long time ago. We can therefore write
+        // with the guarantees that we erase no important data.
+        File.writeAtomic(this.Paths.clean, data, {
+          tmpPath: this.Paths.clean + ".tmp"
+        });
+      } else if (this.state == STATE_RECOVERY) {
+        // At this stage, either $Paths.recovery was written >= 15
+        // seconds ago during this session or we have just started
+        // from $Paths.recovery left from the previous session. Either
+        // way, $Paths.recovery is good. We can move $Path.backup to
+        // $Path.recoveryBackup without erasing a good file with a bad
+        // file.
+        File.writeAtomic(this.Paths.recovery, data, {
+          tmpPath: this.Paths.recovery + ".tmp",
+          backupTo: this.Paths.recoveryBackup
+        });
+      } else {
+        // In other cases, either $Path.recovery is not necessary, or
+        // it doesn't exist or it has been corrupted. Regardless,
+        // don't backup $Path.recovery.
+        File.writeAtomic(this.Paths.recovery, data, {
+          tmpPath: this.Paths.recovery + ".tmp"
+        });
+      }
+
+      telemetry.FX_SESSION_RESTORE_WRITE_FILE_MS = Date.now() - startWriteMs;
+      telemetry.FX_SESSION_RESTORE_FILE_SIZE_BYTES = data.byteLength;
+
+    } catch (ex) {
+      // Don't throw immediately
+      exn = exn || ex;
+    }
+
+    // If necessary, perform an upgrade backup
+    let upgradeBackupComplete = false;
+    if (this.upgradeBackupNeeded
+      && (this.state == STATE_CLEAN || this.state == STATE_UPGRADE_BACKUP)) {
+      try {
+        // If we loaded from `clean`, the file has since then been renamed to `cleanBackup`.
+        let path = this.state == STATE_CLEAN ? this.Paths.cleanBackup : this.Paths.upgradeBackup;
+        File.copy(path, this.Paths.nextUpgradeBackup);
+        this.upgradeBackupNeeded = false;
+        upgradeBackupComplete = true;
+      } catch (ex) {
+        // Don't throw immediately
+        exn = exn || ex;
+      }
+
+      // Find all backups
+      let iterator;
+      let backups = [];  // array that will contain the paths to all upgrade backup
+      let upgradeBackupPrefix = this.Paths.upgradeBackupPrefix;  // access for forEach callback
+
+      try {
+        iterator = new File.DirectoryIterator(this.Paths.backups);
+        iterator.forEach(function (file) {
+          if (file.path.startsWith(upgradeBackupPrefix)) {
+            backups.push(file.path);
+          }
+        }, this);
+      } catch (ex) {
+          // Don't throw immediately
+          exn = exn || ex;
+      } finally {
+        if (iterator) {
+          iterator.close();
+        }
+      }
+
+      // If too many backups exist, delete them
+      if (backups.length > this.maxUpgradeBackups) {
+        // Use alphanumerical sort since dates are in YYYYMMDDHHMMSS format
+        backups.sort().forEach((file, i) => {
+          // remove backup file if it is among the first (n-maxUpgradeBackups) files
+          if (i < backups.length - this.maxUpgradeBackups) {
+            File.remove(file);
+          }
+        });
+      }
+    }
+
+    if (options.performShutdownCleanup && !exn) {
+
+      // During shutdown, if auto-restore is disabled, we need to
+      // remove possibly sensitive data that has been stored purely
+      // for crash recovery. Note that this slightly decreases our
+      // ability to recover from OS-level/hardware-level issue.
+
+      // If an exception was raised, we assume that we still need
+      // these files.
+      File.remove(this.Paths.recoveryBackup);
+      File.remove(this.Paths.recovery);
+    }
+
+    this.state = STATE_RECOVERY;
+
+    if (exn) {
+      throw exn;
+    }
+
+    return {
+      result: {
+        upgradeBackup: upgradeBackupComplete
+      },
+      telemetry: telemetry,
+    };
+  },
+
+  /**
+   * Wipes all files holding session data from disk.
+   */
+  wipe: function () {
+
+    // Don't stop immediately in case of error.
+    let exn = null;
+
+    // Erase main session state file
+    try {
+      File.remove(this.Paths.clean);
+    } catch (ex) {
+      // Don't stop immediately.
+      exn = exn || ex;
+    }
+
+    // Wipe the Session Restore directory
+    try {
+      this._wipeFromDir(this.Paths.backups, null);
+    } catch (ex) {
+      exn = exn || ex;
+    }
+
+    try {
+      File.removeDir(this.Paths.backups);
+    } catch (ex) {
+      exn = exn || ex;
+    }
+
+    // Wipe legacy Ression Restore files from the profile directory
+    try {
+      this._wipeFromDir(OS.Constants.Path.profileDir, "sessionstore.bak");
+    } catch (ex) {
+      exn = exn || ex;
+    }
+
+
+    this.state = STATE_EMPTY;
+    if (exn) {
+      throw exn;
+    }
+
+    return { result: true };
+  },
+
+  /**
+   * Wipe a number of files from a directory.
+   *
+   * @param {string} path The directory.
+   * @param {string|null} prefix If provided, only remove files whose
+   * name starts with a specific prefix.
+   */
+  _wipeFromDir: function(path, prefix) {
+    // Sanity check
+    if (typeof prefix == "undefined" || prefix == "") {
+      throw new TypeError();
+    }
+
+    let exn = null;
+
+    let iterator = new File.DirectoryIterator(path);
+    try {
+      if (!iterator.exists()) {
+        return;
+      }
+      for (let entry in iterator) {
+        if (entry.isDir) {
+          continue;
+        }
+        if (!prefix || entry.name.startsWith(prefix)) {
+          try {
+            File.remove(entry.path);
+          } catch (ex) {
+            // Don't stop immediately
+            exn = exn || ex;
+          }
+        }
+      }
+
+      if (exn) {
+        throw exn;
+      }
+    } finally {
+      iterator.close();
+    }
+  },
+};
+
+function isNoSuchFileEx(aReason) {
+  return aReason instanceof OS.File.Error && aReason.becauseNoSuchFile;
+}
+
+/**
+ * Estimate the number of bytes that a data structure will use on disk
+ * once serialized.
+ */
+function getByteLength(str) {
+  return Encoder.encode(JSON.stringify(str)).byteLength;
+}
diff --git a/application/basilisk/components/sessionstore/SessionWorker.jsm b/application/basilisk/components/sessionstore/SessionWorker.jsm
new file mode 100644
index 0000000000..b26e531ac3
--- /dev/null
+++ b/application/basilisk/components/sessionstore/SessionWorker.jsm
@@ -0,0 +1,25 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this file,
+ * You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+"use strict";
+
+/**
+ * Interface to a dedicated thread handling I/O
+ */
+
+const Cu = Components.utils;
+const Cc = Components.classes;
+const Ci = Components.interfaces;
+const Cr = Components.results;
+
+Cu.import("resource://gre/modules/PromiseWorker.jsm", this);
+Cu.import("resource://gre/modules/osfile.jsm", this);
+
+this.EXPORTED_SYMBOLS = ["SessionWorker"];
+
+this.SessionWorker = new BasePromiseWorker("resource:///modules/sessionstore/SessionWorker.js");
+// As the Session Worker performs I/O, we can receive instances of
+// OS.File.Error, so we need to install a decoder.
+this.SessionWorker.ExceptionHandlers["OS.File.Error"] = OS.File.Error.fromMsg;
+
diff --git a/application/basilisk/components/sessionstore/StartupPerformance.jsm b/application/basilisk/components/sessionstore/StartupPerformance.jsm
new file mode 100644
index 0000000000..d1b77a237e
--- /dev/null
+++ b/application/basilisk/components/sessionstore/StartupPerformance.jsm
@@ -0,0 +1,234 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this file,
+ * You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+"use strict";
+
+this.EXPORTED_SYMBOLS = ["StartupPerformance"];
+
+const { utils: Cu, classes: Cc, interfaces: Ci } = Components;
+
+Cu.import("resource://gre/modules/XPCOMUtils.jsm", this);
+
+XPCOMUtils.defineLazyModuleGetter(this, "Services",
+  "resource://gre/modules/Services.jsm");
+XPCOMUtils.defineLazyModuleGetter(this, "console",
+  "resource://gre/modules/Console.jsm");
+XPCOMUtils.defineLazyModuleGetter(this, "setTimeout",
+  "resource://gre/modules/Timer.jsm");
+XPCOMUtils.defineLazyModuleGetter(this, "clearTimeout",
+  "resource://gre/modules/Timer.jsm");
+XPCOMUtils.defineLazyModuleGetter(this, "Promise",
+  "resource://gre/modules/Promise.jsm");
+
+const COLLECT_RESULTS_AFTER_MS = 10000;
+
+const OBSERVED_TOPICS = ["sessionstore-restoring-on-startup", "sessionstore-initiating-manual-restore"];
+
+this.StartupPerformance = {
+  /**
+   * Once we have finished restoring initial tabs, we broadcast on this topic.
+   */
+  RESTORED_TOPIC: "sessionstore-finished-restoring-initial-tabs",
+
+  // Instant at which we have started restoration (notification "sessionstore-restoring-on-startup")
+  _startTimeStamp: null,
+
+  // Latest instant at which we have finished restoring a tab (DOM event "SSTabRestored")
+  _latestRestoredTimeStamp: null,
+
+  // A promise resolved once we have finished restoring all the startup tabs.
+  _promiseFinished: null,
+
+  // Function `resolve()` for `_promiseFinished`.
+  _resolveFinished: null,
+
+  // A timer
+  _deadlineTimer: null,
+
+  // `true` once the timer has fired
+  _hasFired: false,
+
+  // `true` once we are restored
+  _isRestored: false,
+
+  // Statistics on the session we need to restore.
+  _totalNumberOfEagerTabs: 0,
+  _totalNumberOfTabs: 0,
+  _totalNumberOfWindows: 0,
+
+  init: function() {
+    for (let topic of OBSERVED_TOPICS) {
+      Services.obs.addObserver(this, topic, false);
+    }
+  },
+
+  /**
+   * Return the timestamp at which we finished restoring the latest tab.
+   *
+   * This information is not really interesting until we have finished restoring
+   * tabs.
+   */
+  get latestRestoredTimeStamp() {
+    return this._latestRestoredTimeStamp;
+  },
+
+  /**
+   * `true` once we have finished restoring startup tabs.
+   */
+  get isRestored() {
+    return this._isRestored;
+  },
+
+  // Called when restoration starts.
+  // Record the start timestamp, setup the timer and `this._promiseFinished`.
+  // Behavior is unspecified if there was already an ongoing measure.
+  _onRestorationStarts: function(isAutoRestore) {
+    this._latestRestoredTimeStamp = this._startTimeStamp = Date.now();
+    this._totalNumberOfEagerTabs = 0;
+    this._totalNumberOfTabs = 0;
+    this._totalNumberOfWindows = 0;
+
+    // While we may restore several sessions in a single run of the browser,
+    // that's a very unusual case, and not really worth measuring, so let's
+    // stop listening for further restorations.
+
+    for (let topic of OBSERVED_TOPICS) {
+      Services.obs.removeObserver(this, topic);
+    }
+
+    Services.obs.addObserver(this, "sessionstore-single-window-restored", false);
+    this._promiseFinished = new Promise(resolve => {
+      this._resolveFinished = resolve;
+    });
+    this._promiseFinished.then(() => {
+      try {
+        this._isRestored = true;
+        Services.obs.notifyObservers(null, this.RESTORED_TOPIC, "");
+
+        if (this._latestRestoredTimeStamp == this._startTimeStamp) {
+          // Apparently, we haven't restored any tab.
+          return;
+        }
+
+        // Once we are done restoring tabs, update Telemetry.
+        let histogramName = isAutoRestore ?
+          "FX_SESSION_RESTORE_AUTO_RESTORE_DURATION_UNTIL_EAGER_TABS_RESTORED_MS" :
+          "FX_SESSION_RESTORE_MANUAL_RESTORE_DURATION_UNTIL_EAGER_TABS_RESTORED_MS";
+        let histogram = Services.telemetry.getHistogramById(histogramName);
+        let delta = this._latestRestoredTimeStamp - this._startTimeStamp;
+        histogram.add(delta);
+
+        Services.telemetry.getHistogramById("FX_SESSION_RESTORE_NUMBER_OF_EAGER_TABS_RESTORED").add(this._totalNumberOfEagerTabs);
+        Services.telemetry.getHistogramById("FX_SESSION_RESTORE_NUMBER_OF_TABS_RESTORED").add(this._totalNumberOfTabs);
+        Services.telemetry.getHistogramById("FX_SESSION_RESTORE_NUMBER_OF_WINDOWS_RESTORED").add(this._totalNumberOfWindows);
+
+        // Reset
+        this._startTimeStamp = null;
+     } catch (ex) {
+        console.error("StartupPerformance: error after resolving promise", ex);
+      }
+    });
+  },
+
+  _startTimer: function() {
+    if (this._hasFired) {
+      return;
+    }
+    if (this._deadlineTimer) {
+      clearTimeout(this._deadlineTimer);
+    }
+    this._deadlineTimer = setTimeout(() => {
+      try {
+        this._resolveFinished();
+      } catch (ex) {
+        console.error("StartupPerformance: Error in timeout handler", ex);
+      } finally {
+        // Clean up.
+        this._deadlineTimer = null;
+        this._hasFired = true;
+        this._resolveFinished = null;
+        Services.obs.removeObserver(this, "sessionstore-single-window-restored");
+      }
+    }, COLLECT_RESULTS_AFTER_MS);
+  },
+
+  observe: function(subject, topic, details) {
+    try {
+      switch (topic) {
+        case "sessionstore-restoring-on-startup":
+          this._onRestorationStarts(true);
+          break;
+        case "sessionstore-initiating-manual-restore":
+          this._onRestorationStarts(false);
+          break;
+        case "sessionstore-single-window-restored": {
+          // Session Restore has just opened a window with (initially empty) tabs.
+          // Some of these tabs will be restored eagerly, while others will be
+          // restored on demand. The process becomes usable only when all windows
+          // have finished restored their eager tabs.
+          //
+          // While it would be possible to track the restoration of each tab
+          // from within SessionRestore to determine exactly when the process
+          // becomes usable, experience shows that this is too invasive. Rather,
+          // we employ the following heuristic:
+          // - we maintain a timer of `COLLECT_RESULTS_AFTER_MS` that we expect
+          //   will be triggered only once all tabs have been restored;
+          // - whenever we restore a new window (hence a bunch of eager tabs),
+          //   we postpone the timer to ensure that the new eager tabs have
+          //   `COLLECT_RESULTS_AFTER_MS` to be restored;
+          // - whenever a tab is restored, we update
+          //   `this._latestRestoredTimeStamp`;
+          // - after `COLLECT_RESULTS_AFTER_MS`, we collect the final version
+          //   of `this._latestRestoredTimeStamp`, and use it to determine the
+          //   entire duration of the collection.
+          //
+          // Note that this heuristic may be inaccurate if a user clicks
+          // immediately on a restore-on-demand tab before the end of
+          // `COLLECT_RESULTS_AFTER_MS`. We assume that this will not
+          // affect too much the results.
+          //
+          // Reset the delay, to give the tabs a little (more) time to restore.
+          this._startTimer();
+
+          this._totalNumberOfWindows += 1;
+
+          // Observe the restoration of all tabs. We assume that all tabs of this
+          // window will have been restored before `COLLECT_RESULTS_AFTER_MS`.
+          // The last call to `observer` will let us determine how long it took
+          // to reach that point.
+          let win = subject;
+
+          let observer = (event) => {
+            // We don't care about tab restorations that are due to
+            // a browser flipping from out-of-main-process to in-main-process
+            // or vice-versa. We only care about restorations that are due
+            // to the user switching to a lazily restored tab, or for tabs
+            // that are restoring eagerly.
+            if (!event.detail.isRemotenessUpdate) {
+              this._latestRestoredTimeStamp = Date.now();
+              this._totalNumberOfEagerTabs += 1;
+            }
+          };
+          win.gBrowser.tabContainer.addEventListener("SSTabRestored", observer);
+          this._totalNumberOfTabs += win.gBrowser.tabContainer.itemCount;
+
+          // Once we have finished collecting the results, clean up the observers.
+          this._promiseFinished.then(() => {
+            if (!win.gBrowser.tabContainer) {
+              // May be undefined during shutdown and/or some tests.
+              return;
+            }
+            win.gBrowser.tabContainer.removeEventListener("SSTabRestored", observer);
+          });
+        }
+        break;
+        default:
+          throw new Error(`Unexpected topic ${topic}`);
+      }
+    } catch (ex) {
+      console.error("StartupPerformance error", ex, ex.stack);
+      throw ex;
+    }
+  }
+};
diff --git a/application/basilisk/components/sessionstore/TabAttributes.jsm b/application/basilisk/components/sessionstore/TabAttributes.jsm
new file mode 100644
index 0000000000..8a29680f47
--- /dev/null
+++ b/application/basilisk/components/sessionstore/TabAttributes.jsm
@@ -0,0 +1,74 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this file,
+ * You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+"use strict";
+
+this.EXPORTED_SYMBOLS = ["TabAttributes"];
+
+// We never want to directly read or write these attributes.
+// 'image' should not be accessed directly but handled by using the
+//         gBrowser.getIcon()/setIcon() methods.
+// 'muted' should not be accessed directly but handled by using the
+//         tab.linkedBrowser.audioMuted/toggleMuteAudio methods.
+// 'pending' is used internal by sessionstore and managed accordingly.
+// 'iconLoadingPrincipal' is same as 'image' that it should be handled by
+//                        using the gBrowser.getIcon()/setIcon() methods.
+const ATTRIBUTES_TO_SKIP = new Set(["image", "muted", "pending", "iconLoadingPrincipal"]);
+
+// A set of tab attributes to persist. We will read a given list of tab
+// attributes when collecting tab data and will re-set those attributes when
+// the given tab data is restored to a new tab.
+this.TabAttributes = Object.freeze({
+  persist: function (name) {
+    return TabAttributesInternal.persist(name);
+  },
+
+  get: function (tab) {
+    return TabAttributesInternal.get(tab);
+  },
+
+  set: function (tab, data = {}) {
+    TabAttributesInternal.set(tab, data);
+  }
+});
+
+var TabAttributesInternal = {
+  _attrs: new Set(),
+
+  persist: function (name) {
+    if (this._attrs.has(name) || ATTRIBUTES_TO_SKIP.has(name)) {
+      return false;
+    }
+
+    this._attrs.add(name);
+    return true;
+  },
+
+  get: function (tab) {
+    let data = {};
+
+    for (let name of this._attrs) {
+      if (tab.hasAttribute(name)) {
+        data[name] = tab.getAttribute(name);
+      }
+    }
+
+    return data;
+  },
+
+  set: function (tab, data = {}) {
+    // Clear attributes.
+    for (let name of this._attrs) {
+      tab.removeAttribute(name);
+    }
+
+    // Set attributes.
+    for (let name in data) {
+      if (!ATTRIBUTES_TO_SKIP.has(name)) {
+        tab.setAttribute(name, data[name]);
+      }
+    }
+  }
+};
+
diff --git a/application/basilisk/components/sessionstore/TabState.jsm b/application/basilisk/components/sessionstore/TabState.jsm
new file mode 100644
index 0000000000..f22c52fe33
--- /dev/null
+++ b/application/basilisk/components/sessionstore/TabState.jsm
@@ -0,0 +1,196 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this file,
+ * You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+"use strict";
+
+this.EXPORTED_SYMBOLS = ["TabState"];
+
+const Cu = Components.utils;
+
+Cu.import("resource://gre/modules/XPCOMUtils.jsm", this);
+
+XPCOMUtils.defineLazyModuleGetter(this, "PrivacyFilter",
+  "resource:///modules/sessionstore/PrivacyFilter.jsm");
+XPCOMUtils.defineLazyModuleGetter(this, "TabStateCache",
+  "resource:///modules/sessionstore/TabStateCache.jsm");
+XPCOMUtils.defineLazyModuleGetter(this, "TabAttributes",
+  "resource:///modules/sessionstore/TabAttributes.jsm");
+XPCOMUtils.defineLazyModuleGetter(this, "Utils",
+  "resource://gre/modules/sessionstore/Utils.jsm");
+
+/**
+ * Module that contains tab state collection methods.
+ */
+this.TabState = Object.freeze({
+  update: function (browser, data) {
+    TabStateInternal.update(browser, data);
+  },
+
+  collect: function (tab) {
+    return TabStateInternal.collect(tab);
+  },
+
+  clone: function (tab) {
+    return TabStateInternal.clone(tab);
+  },
+
+  copyFromCache(browser, tabData, options) {
+    TabStateInternal.copyFromCache(browser, tabData, options);
+  },
+});
+
+var TabStateInternal = {
+  /**
+   * Processes a data update sent by the content script.
+   */
+  update: function (browser, {data}) {
+    TabStateCache.update(browser, data);
+  },
+
+  /**
+   * Collect data related to a single tab, synchronously.
+   *
+   * @param tab
+   *        tabbrowser tab
+   *
+   * @returns {TabData} An object with the data for this tab.  If the
+   * tab has not been invalidated since the last call to
+   * collect(aTab), the same object is returned.
+   */
+  collect: function (tab) {
+    return this._collectBaseTabData(tab);
+  },
+
+  /**
+   * Collect data related to a single tab, including private data.
+   * Use with caution.
+   *
+   * @param tab
+   *        tabbrowser tab
+   *
+   * @returns {object} An object with the data for this tab. This data is never
+   *                   cached, it will always be read from the tab and thus be
+   *                   up-to-date.
+   */
+  clone: function (tab) {
+    return this._collectBaseTabData(tab, {includePrivateData: true});
+  },
+
+  /**
+   * Collects basic tab data for a given tab.
+   *
+   * @param tab
+   *        tabbrowser tab
+   * @param options (object)
+   *        {includePrivateData: true} to always include private data
+   *
+   * @returns {object} An object with the basic data for this tab.
+   */
+  _collectBaseTabData: function (tab, options) {
+    let tabData = { entries: [], lastAccessed: tab.lastAccessed };
+    let browser = tab.linkedBrowser;
+
+    if (tab.pinned) {
+      tabData.pinned = true;
+    }
+
+    tabData.hidden = tab.hidden;
+
+    if (browser.audioMuted) {
+      tabData.muted = true;
+      tabData.muteReason = tab.muteReason;
+    }
+
+    // Save tab attributes.
+    tabData.attributes = TabAttributes.get(tab);
+
+    if (tab.__SS_extdata) {
+      tabData.extData = tab.__SS_extdata;
+    }
+
+    // Copy data from the tab state cache only if the tab has fully finished
+    // restoring. We don't want to overwrite data contained in __SS_data.
+    this.copyFromCache(browser, tabData, options);
+
+    // After copyFromCache() was called we check for properties that are kept
+    // in the cache only while the tab is pending or restoring. Once that
+    // happened those properties will be removed from the cache and will
+    // be read from the tab/browser every time we collect data.
+
+    // Store the tab icon.
+    if (!("image" in tabData)) {
+      let tabbrowser = tab.ownerGlobal.gBrowser;
+      tabData.image = tabbrowser.getIcon(tab);
+    }
+
+    // Store the serialized contentPrincipal of this tab to use for the icon.
+    if (!("iconLoadingPrincipal" in tabData)) {
+      tabData.iconLoadingPrincipal = Utils.serializePrincipal(browser.contentPrincipal);
+    }
+
+    // If there is a userTypedValue set, then either the user has typed something
+    // in the URL bar, or a new tab was opened with a URI to load.
+    // If so, we also track whether we were still in the process of loading something.
+    if (!("userTypedValue" in tabData) && browser.userTypedValue) {
+      tabData.userTypedValue = browser.userTypedValue;
+      // We always used to keep track of the loading state as an integer, where
+      // '0' indicated the user had typed since the last load (or no load was
+      // ongoing), and any positive value indicated we had started a load since
+      // the last time the user typed in the URL bar. Mimic this to keep the
+      // session store representation in sync, even though we now represent this
+      // more explicitly:
+      tabData.userTypedClear = browser.didStartLoadSinceLastUserTyping() ? 1 : 0;
+    }
+
+    return tabData;
+  },
+
+  /**
+   * Copy data for the given |browser| from the cache to |tabData|.
+   *
+   * @param browser (xul:browser)
+   *        The browser belonging to the given |tabData| object.
+   * @param tabData (object)
+   *        The tab data belonging to the given |tab|.
+   * @param options (object)
+   *        {includePrivateData: true} to always include private data
+   */
+  copyFromCache(browser, tabData, options = {}) {
+    let data = TabStateCache.get(browser);
+    if (!data) {
+      return;
+    }
+
+    // The caller may explicitly request to omit privacy checks.
+    let includePrivateData = options && options.includePrivateData;
+    let isPinned = !!tabData.pinned;
+
+    for (let key of Object.keys(data)) {
+      let value = data[key];
+
+      // Filter sensitive data according to the current privacy level.
+      if (!includePrivateData) {
+        if (key === "storage") {
+          value = PrivacyFilter.filterSessionStorageData(value);
+        } else if (key === "formdata") {
+          value = PrivacyFilter.filterFormData(value);
+        }
+      }
+
+      if (key === "history") {
+        tabData.entries = value.entries;
+
+        if (value.hasOwnProperty("userContextId")) {
+          tabData.userContextId = value.userContextId;
+        }
+
+        if (value.hasOwnProperty("index")) {
+          tabData.index = value.index;
+        }
+      } else {
+        tabData[key] = value;
+      }
+    }
+  }
+};
diff --git a/application/basilisk/components/sessionstore/TabStateCache.jsm b/application/basilisk/components/sessionstore/TabStateCache.jsm
new file mode 100644
index 0000000000..9bed315a03
--- /dev/null
+++ b/application/basilisk/components/sessionstore/TabStateCache.jsm
@@ -0,0 +1,163 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this file,
+ * You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+"use strict";
+
+this.EXPORTED_SYMBOLS = ["TabStateCache"];
+
+/**
+ * A cache for tabs data.
+ *
+ * This cache implements a weak map from tabs (as XUL elements)
+ * to tab data (as objects).
+ *
+ * Note that we should never cache private data, as:
+ * - that data is used very seldom by SessionStore;
+ * - caching private data in addition to public data is memory consuming.
+ */
+this.TabStateCache = Object.freeze({
+  /**
+   * Retrieves cached data for a given |tab| or associated |browser|.
+   *
+   * @param browserOrTab (xul:tab or xul:browser)
+   *        The tab or browser to retrieve cached data for.
+   * @return (object)
+   *         The cached data stored for the given |tab|
+   *         or associated |browser|.
+   */
+  get: function (browserOrTab) {
+    return TabStateCacheInternal.get(browserOrTab);
+  },
+
+  /**
+   * Updates cached data for a given |tab| or associated |browser|.
+   *
+   * @param browserOrTab (xul:tab or xul:browser)
+   *        The tab or browser belonging to the given tab data.
+   * @param newData (object)
+   *        The new data to be stored for the given |tab|
+   *        or associated |browser|.
+   */
+  update: function (browserOrTab, newData) {
+    TabStateCacheInternal.update(browserOrTab, newData);
+  }
+});
+
+var TabStateCacheInternal = {
+  _data: new WeakMap(),
+
+  /**
+   * Retrieves cached data for a given |tab| or associated |browser|.
+   *
+   * @param browserOrTab (xul:tab or xul:browser)
+   *        The tab or browser to retrieve cached data for.
+   * @return (object)
+   *         The cached data stored for the given |tab|
+   *         or associated |browser|.
+   */
+  get: function (browserOrTab) {
+    return this._data.get(browserOrTab.permanentKey);
+  },
+
+  /**
+   * Helper function used by update (see below). For message size
+   * optimization sometimes we don't update the whole session storage
+   * only the values that have been changed.
+   *
+   * @param data (object)
+   *        The cached data where we want to update the changes.
+   * @param change (object)
+   *        The actual changed values per domain.
+   */
+  updatePartialStorageChange: function (data, change) {
+    if (!data.storage) {
+      data.storage = {};
+    }
+
+    let storage = data.storage;
+    for (let domain of Object.keys(change)) {
+      for (let key of Object.keys(change[domain])) {
+        let value = change[domain][key];
+        if (value === null) {
+          if (storage[domain] && storage[domain][key]) {
+            delete storage[domain][key];
+          }
+        } else {
+          if (!storage[domain]) {
+            storage[domain] = {};
+          }
+          storage[domain][key] = value;
+        }
+      }
+    }
+  },
+
+  /**
+   * Helper function used by update (see below). For message size
+   * optimization sometimes we don't update the whole browser history
+   * only the current index and the tail of the history from a certain
+   * index (specified by change.fromIdx)
+   *
+   * @param data (object)
+   *        The cached data where we want to update the changes.
+   * @param change (object)
+   *        Object containing the tail of the history array, and
+   *        some additional metadata.
+   */
+  updatePartialHistoryChange: function (data, change) {
+    const kLastIndex = Number.MAX_SAFE_INTEGER - 1;
+
+    if (!data.history) {
+      data.history = { entries: [] };
+    }
+
+    let history = data.history;
+    for (let key of Object.keys(change)) {
+      if (key == "entries") {
+        if (change.fromIdx != kLastIndex) {
+          history.entries.splice(change.fromIdx + 1);
+          while (change.entries.length) {
+            history.entries.push(change.entries.shift());
+          }
+        }
+      } else if (key != "fromIndex") {
+        history[key] = change[key];
+      }
+    }
+  },
+
+  /**
+   * Updates cached data for a given |tab| or associated |browser|.
+   *
+   * @param browserOrTab (xul:tab or xul:browser)
+   *        The tab or browser belonging to the given tab data.
+   * @param newData (object)
+   *        The new data to be stored for the given |tab|
+   *        or associated |browser|.
+   */
+  update: function (browserOrTab, newData) {
+    let data = this._data.get(browserOrTab.permanentKey) || {};
+
+    for (let key of Object.keys(newData)) {
+      if (key == "storagechange") {
+        this.updatePartialStorageChange(data, newData.storagechange);
+        continue;
+      }
+
+      if (key == "historychange") {
+        this.updatePartialHistoryChange(data, newData.historychange);
+        continue;
+      }
+
+      let value = newData[key];
+      if (value === null) {
+        delete data[key];
+      } else {
+        data[key] = value;
+      }
+    }
+
+    this._data.set(browserOrTab.permanentKey, data);
+  }
+};
diff --git a/application/basilisk/components/sessionstore/TabStateFlusher.jsm b/application/basilisk/components/sessionstore/TabStateFlusher.jsm
new file mode 100644
index 0000000000..6397efe9d8
--- /dev/null
+++ b/application/basilisk/components/sessionstore/TabStateFlusher.jsm
@@ -0,0 +1,184 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this file,
+ * You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+"use strict";
+
+this.EXPORTED_SYMBOLS = ["TabStateFlusher"];
+
+const Cu = Components.utils;
+
+Cu.import("resource://gre/modules/Promise.jsm", this);
+
+/**
+ * A module that enables async flushes. Updates from frame scripts are
+ * throttled to be sent only once per second. If an action wants a tab's latest
+ * state without waiting for a second then it can request an async flush and
+ * wait until the frame scripts reported back. At this point the parent has the
+ * latest data and the action can continue.
+ */
+this.TabStateFlusher = Object.freeze({
+  /**
+   * Requests an async flush for the given browser. Returns a promise that will
+   * resolve when we heard back from the content process and the parent has
+   * all the latest data.
+   */
+  flush(browser) {
+    return TabStateFlusherInternal.flush(browser);
+  },
+
+  /**
+   * Requests an async flush for all browsers of a given window. Returns a Promise
+   * that will resolve when we've heard back from all browsers.
+   */
+  flushWindow(window) {
+    return TabStateFlusherInternal.flushWindow(window);
+  },
+
+  /**
+   * Resolves the flush request with the given flush ID.
+   *
+   * @param browser (<xul:browser>)
+   *        The browser for which the flush is being resolved.
+   * @param flushID (int)
+   *        The ID of the flush that was sent to the browser.
+   * @param success (bool, optional)
+   *        Whether or not the flush succeeded.
+   * @param message (string, optional)
+   *        An error message that will be sent to the Console in the
+   *        event that a flush failed.
+   */
+  resolve(browser, flushID, success=true, message="") {
+    TabStateFlusherInternal.resolve(browser, flushID, success, message);
+  },
+
+  /**
+   * Resolves all active flush requests for a given browser. This should be
+   * used when the content process crashed or the final update message was
+   * seen. In those cases we can't guarantee to ever hear back from the frame
+   * script so we just resolve all requests instead of discarding them.
+   *
+   * @param browser (<xul:browser>)
+   *        The browser for which all flushes are being resolved.
+   * @param success (bool, optional)
+   *        Whether or not the flushes succeeded.
+   * @param message (string, optional)
+   *        An error message that will be sent to the Console in the
+   *        event that the flushes failed.
+   */
+  resolveAll(browser, success=true, message="") {
+    TabStateFlusherInternal.resolveAll(browser, success, message);
+  }
+});
+
+var TabStateFlusherInternal = {
+  // Stores the last request ID.
+  _lastRequestID: 0,
+
+  // A map storing all active requests per browser.
+  _requests: new WeakMap(),
+
+  /**
+   * Requests an async flush for the given browser. Returns a promise that will
+   * resolve when we heard back from the content process and the parent has
+   * all the latest data.
+   */
+  flush(browser) {
+    let id = ++this._lastRequestID;
+    let mm = browser.messageManager;
+    mm.sendAsyncMessage("SessionStore:flush", {id});
+
+    // Retrieve active requests for given browser.
+    let permanentKey = browser.permanentKey;
+    let perBrowserRequests = this._requests.get(permanentKey) || new Map();
+
+    return new Promise(resolve => {
+      // Store resolve() so that we can resolve the promise later.
+      perBrowserRequests.set(id, resolve);
+
+      // Update the flush requests stored per browser.
+      this._requests.set(permanentKey, perBrowserRequests);
+    });
+  },
+
+  /**
+   * Requests an async flush for all browsers of a given window. Returns a Promise
+   * that will resolve when we've heard back from all browsers.
+   */
+  flushWindow(window) {
+    let browsers = window.gBrowser.browsers;
+    let promises = browsers.map((browser) => this.flush(browser));
+    return Promise.all(promises);
+  },
+
+  /**
+   * Resolves the flush request with the given flush ID.
+   *
+   * @param browser (<xul:browser>)
+   *        The browser for which the flush is being resolved.
+   * @param flushID (int)
+   *        The ID of the flush that was sent to the browser.
+   * @param success (bool, optional)
+   *        Whether or not the flush succeeded.
+   * @param message (string, optional)
+   *        An error message that will be sent to the Console in the
+   *        event that a flush failed.
+   */
+  resolve(browser, flushID, success=true, message="") {
+    // Nothing to do if there are no pending flushes for the given browser.
+    if (!this._requests.has(browser.permanentKey)) {
+      return;
+    }
+
+    // Retrieve active requests for given browser.
+    let perBrowserRequests = this._requests.get(browser.permanentKey);
+    if (!perBrowserRequests.has(flushID)) {
+      return;
+    }
+
+    if (!success) {
+      Cu.reportError("Failed to flush browser: " + message);
+    }
+
+    // Resolve the request with the given id.
+    let resolve = perBrowserRequests.get(flushID);
+    perBrowserRequests.delete(flushID);
+    resolve(success);
+  },
+
+  /**
+   * Resolves all active flush requests for a given browser. This should be
+   * used when the content process crashed or the final update message was
+   * seen. In those cases we can't guarantee to ever hear back from the frame
+   * script so we just resolve all requests instead of discarding them.
+   *
+   * @param browser (<xul:browser>)
+   *        The browser for which all flushes are being resolved.
+   * @param success (bool, optional)
+   *        Whether or not the flushes succeeded.
+   * @param message (string, optional)
+   *        An error message that will be sent to the Console in the
+   *        event that the flushes failed.
+   */
+  resolveAll(browser, success=true, message="") {
+    // Nothing to do if there are no pending flushes for the given browser.
+    if (!this._requests.has(browser.permanentKey)) {
+      return;
+    }
+
+    // Retrieve active requests for given browser.
+    let perBrowserRequests = this._requests.get(browser.permanentKey);
+
+    if (!success) {
+      Cu.reportError("Failed to flush browser: " + message);
+    }
+
+    // Resolve all requests.
+    for (let resolve of perBrowserRequests.values()) {
+      resolve(success);
+    }
+
+    // Clear active requests.
+    perBrowserRequests.clear();
+  }
+};
diff --git a/application/basilisk/components/sessionstore/content/aboutSessionRestore.js b/application/basilisk/components/sessionstore/content/aboutSessionRestore.js
new file mode 100644
index 0000000000..8f265235d7
--- /dev/null
+++ b/application/basilisk/components/sessionstore/content/aboutSessionRestore.js
@@ -0,0 +1,373 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+"use strict";
+
+var {classes: Cc, interfaces: Ci, utils: Cu} = Components;
+
+Cu.import("resource://gre/modules/Services.jsm");
+Cu.import("resource://gre/modules/XPCOMUtils.jsm");
+XPCOMUtils.defineLazyModuleGetter(this, "AppConstants",
+  "resource://gre/modules/AppConstants.jsm");
+
+var gStateObject;
+var gTreeData;
+
+// Page initialization
+
+window.onload = function() {
+  // pages used by this script may have a link that needs to be updated to
+  // the in-product link.
+  let anchor = document.getElementById("linkMoreTroubleshooting");
+  if (anchor) {
+    let baseURL = Services.urlFormatter.formatURLPref("app.support.baseURL");
+    anchor.setAttribute("href", baseURL + "troubleshooting");
+  }
+
+  // wire up click handlers for the radio buttons if they exist.
+  for (let radioId of ["radioRestoreAll", "radioRestoreChoose"]) {
+    let button = document.getElementById(radioId);
+    if (button) {
+      button.addEventListener("click", updateTabListVisibility);
+    }
+  }
+
+  // the crashed session state is kept inside a textbox so that SessionStore picks it up
+  // (for when the tab is closed or the session crashes right again)
+  var sessionData = document.getElementById("sessionData");
+  if (!sessionData.value) {
+    document.getElementById("errorTryAgain").disabled = true;
+    return;
+  }
+
+  try {
+    gStateObject = JSON.parse(sessionData.value);
+  } catch (e) {
+    Cu.reportError(e);
+  }
+
+  // make sure the data is tracked to be restored in case of a subsequent crash
+  var event = document.createEvent("UIEvents");
+  event.initUIEvent("input", true, true, window, 0);
+  sessionData.dispatchEvent(event);
+
+  initTreeView();
+
+  document.getElementById("errorTryAgain").focus();
+};
+
+function isTreeViewVisible() {
+  let tabList = document.querySelector(".tree-container");
+  return tabList.hasAttribute("available");
+}
+
+function initTreeView() {
+  // If we aren't visible we initialize as we are made visible (and it's OK
+  // to initialize multiple times)
+  if (!isTreeViewVisible()) {
+    return;
+  }
+  var tabList = document.getElementById("tabList");
+  var winLabel = tabList.getAttribute("_window_label");
+
+  gTreeData = [];
+  if (gStateObject) {
+    gStateObject.windows.forEach(function(aWinData, aIx) {
+      var winState = {
+        label: winLabel.replace("%S", (aIx + 1)),
+        open: true,
+        checked: true,
+        ix: aIx
+      };
+      winState.tabs = aWinData.tabs.map(function(aTabData) {
+        var entry = aTabData.entries[aTabData.index - 1] || { url: "about:blank" };
+        var iconURL = aTabData.image || null;
+        // don't initiate a connection just to fetch a favicon (see bug 462863)
+        if (/^https?:/.test(iconURL))
+          iconURL = "moz-anno:favicon:" + iconURL;
+        return {
+          label: entry.title || entry.url,
+          checked: true,
+          src: iconURL,
+          parent: winState
+        };
+      });
+      gTreeData.push(winState);
+      for (let tab of winState.tabs)
+        gTreeData.push(tab);
+    }, this);
+  }
+
+  tabList.view = treeView;
+  tabList.view.selection.select(0);
+}
+
+// User actions
+function updateTabListVisibility() {
+  let tabList = document.querySelector(".tree-container");
+  let container = document.querySelector(".container");
+  if (document.getElementById("radioRestoreChoose").checked) {
+    tabList.setAttribute("available", "true");
+    container.classList.add("restore-chosen");
+  } else {
+    tabList.removeAttribute("available");
+    container.classList.remove("restore-chosen");
+  }
+  initTreeView();
+}
+
+function restoreSession() {
+  Services.obs.notifyObservers(null, "sessionstore-initiating-manual-restore", "");
+  document.getElementById("errorTryAgain").disabled = true;
+
+  if (isTreeViewVisible()) {
+    if (!gTreeData.some(aItem => aItem.checked)) {
+      // This should only be possible when we have no "cancel" button, and thus
+      // the "Restore session" button always remains enabled.  In that case and
+      // when nothing is selected, we just want a new session.
+      startNewSession();
+      return;
+    }
+
+    // remove all unselected tabs from the state before restoring it
+    var ix = gStateObject.windows.length - 1;
+    for (var t = gTreeData.length - 1; t >= 0; t--) {
+      if (treeView.isContainer(t)) {
+        if (gTreeData[t].checked === 0)
+          // this window will be restored partially
+          gStateObject.windows[ix].tabs =
+            gStateObject.windows[ix].tabs.filter((aTabData, aIx) =>
+                                                   gTreeData[t].tabs[aIx].checked);
+        else if (!gTreeData[t].checked)
+          // this window won't be restored at all
+          gStateObject.windows.splice(ix, 1);
+        ix--;
+      }
+    }
+  }
+  var stateString = JSON.stringify(gStateObject);
+
+  var ss = Cc["@mozilla.org/browser/sessionstore;1"].getService(Ci.nsISessionStore);
+  var top = getBrowserWindow();
+
+  // if there's only this page open, reuse the window for restoring the session
+  if (top.gBrowser.tabs.length == 1) {
+    ss.setWindowState(top, stateString, true);
+    return;
+  }
+
+  // restore the session into a new window and close the current tab
+  var newWindow = top.openDialog(top.location, "_blank", "chrome,dialog=no,all");
+
+  var obs = Cc["@mozilla.org/observer-service;1"].getService(Ci.nsIObserverService);
+  obs.addObserver(function observe(win, topic) {
+    if (win != newWindow) {
+      return;
+    }
+
+    obs.removeObserver(observe, topic);
+    ss.setWindowState(newWindow, stateString, true);
+
+    var tabbrowser = top.gBrowser;
+    var tabIndex = tabbrowser.getBrowserIndexForDocument(document);
+    tabbrowser.removeTab(tabbrowser.tabs[tabIndex]);
+  }, "browser-delayed-startup-finished", false);
+}
+
+function startNewSession() {
+  var prefBranch = Cc["@mozilla.org/preferences-service;1"].getService(Ci.nsIPrefBranch);
+  if (prefBranch.getIntPref("browser.startup.page") == 0)
+    getBrowserWindow().gBrowser.loadURI("about:blank");
+  else
+    getBrowserWindow().BrowserHome();
+}
+
+function onListClick(aEvent) {
+  // don't react to right-clicks
+  if (aEvent.button == 2)
+    return;
+
+  if (!treeView.treeBox) {
+    return;
+  }
+  var cell = treeView.treeBox.getCellAt(aEvent.clientX, aEvent.clientY);
+  if (cell.col) {
+    // Restore this specific tab in the same window for middle/double/accel clicking
+    // on a tab's title.
+    let accelKey = AppConstants.platform == "macosx" ?
+                   aEvent.metaKey :
+                   aEvent.ctrlKey;
+    if ((aEvent.button == 1 || aEvent.button == 0 && aEvent.detail == 2 || accelKey) &&
+        cell.col.id == "title" &&
+        !treeView.isContainer(cell.row)) {
+      restoreSingleTab(cell.row, aEvent.shiftKey);
+      aEvent.stopPropagation();
+    }
+    else if (cell.col.id == "restore")
+      toggleRowChecked(cell.row);
+  }
+}
+
+function onListKeyDown(aEvent) {
+  switch (aEvent.keyCode)
+  {
+  case KeyEvent.DOM_VK_SPACE:
+    toggleRowChecked(document.getElementById("tabList").currentIndex);
+    // Prevent page from scrolling on the space key.
+    aEvent.preventDefault();
+    break;
+  case KeyEvent.DOM_VK_RETURN:
+    var ix = document.getElementById("tabList").currentIndex;
+    if (aEvent.ctrlKey && !treeView.isContainer(ix))
+      restoreSingleTab(ix, aEvent.shiftKey);
+    break;
+  }
+}
+
+// Helper functions
+
+function getBrowserWindow() {
+  return window.QueryInterface(Ci.nsIInterfaceRequestor).getInterface(Ci.nsIWebNavigation)
+               .QueryInterface(Ci.nsIDocShellTreeItem).rootTreeItem
+               .QueryInterface(Ci.nsIInterfaceRequestor).getInterface(Ci.nsIDOMWindow);
+}
+
+function toggleRowChecked(aIx) {
+  function isChecked(aItem) {
+    return aItem.checked;
+  }
+
+  var item = gTreeData[aIx];
+  item.checked = !item.checked;
+  treeView.treeBox.invalidateRow(aIx);
+
+  if (treeView.isContainer(aIx)) {
+    // (un)check all tabs of this window as well
+    for (let tab of item.tabs) {
+      tab.checked = item.checked;
+      treeView.treeBox.invalidateRow(gTreeData.indexOf(tab));
+    }
+  }
+  else {
+    // update the window's checkmark as well (0 means "partially checked")
+    item.parent.checked = item.parent.tabs.every(isChecked) ? true :
+                          item.parent.tabs.some(isChecked) ? 0 : false;
+    treeView.treeBox.invalidateRow(gTreeData.indexOf(item.parent));
+  }
+
+  // we only disable the button when there's no cancel button.
+  if (document.getElementById("errorCancel")) {
+    document.getElementById("errorTryAgain").disabled = !gTreeData.some(isChecked);
+  }
+}
+
+function restoreSingleTab(aIx, aShifted) {
+  var tabbrowser = getBrowserWindow().gBrowser;
+  var newTab = tabbrowser.addTab();
+  var item = gTreeData[aIx];
+
+  var ss = Cc["@mozilla.org/browser/sessionstore;1"].getService(Ci.nsISessionStore);
+  var tabState = gStateObject.windows[item.parent.ix]
+                             .tabs[aIx - gTreeData.indexOf(item.parent) - 1];
+  // ensure tab would be visible on the tabstrip.
+  tabState.hidden = false;
+  ss.setTabState(newTab, JSON.stringify(tabState));
+
+  // respect the preference as to whether to select the tab (the Shift key inverses)
+  var prefBranch = Cc["@mozilla.org/preferences-service;1"].getService(Ci.nsIPrefBranch);
+  if (prefBranch.getBoolPref("browser.tabs.loadInBackground") != !aShifted)
+    tabbrowser.selectedTab = newTab;
+}
+
+// Tree controller
+
+var treeView = {
+  treeBox: null,
+  selection: null,
+
+  get rowCount()                     { return gTreeData.length; },
+  setTree: function(treeBox)         { this.treeBox = treeBox; },
+  getCellText: function(idx, column) { return gTreeData[idx].label; },
+  isContainer: function(idx)         {
+    return gTreeData[idx] ? "open" in gTreeData[idx] : false;
+  },
+  getCellValue: function(idx, column){ return gTreeData[idx].checked; },
+  isContainerOpen: function(idx)     { return gTreeData[idx].open; },
+  isContainerEmpty: function(idx)    { return false; },
+  isSeparator: function(idx)         { return false; },
+  isSorted: function()               { return false; },
+  isEditable: function(idx, column)  { return false; },
+  canDrop: function(idx, orientation, dt) { return false; },
+  getLevel: function(idx)            { return this.isContainer(idx) ? 0 : 1; },
+
+  getParentIndex: function(idx) {
+    if (!this.isContainer(idx))
+      for (var t = idx - 1; t >= 0 ; t--)
+        if (this.isContainer(t))
+          return t;
+    return -1;
+  },
+
+  hasNextSibling: function(idx, after) {
+    var thisLevel = this.getLevel(idx);
+    for (var t = after + 1; t < gTreeData.length; t++)
+      if (this.getLevel(t) <= thisLevel)
+        return this.getLevel(t) == thisLevel;
+    return false;
+  },
+
+  toggleOpenState: function(idx) {
+    if (!this.isContainer(idx))
+      return;
+    var item = gTreeData[idx];
+    if (item.open) {
+      // remove this window's tab rows from the view
+      var thisLevel = this.getLevel(idx);
+      for (var t = idx + 1; t < gTreeData.length && this.getLevel(t) > thisLevel; t++);
+      var deletecount = t - idx - 1;
+      gTreeData.splice(idx + 1, deletecount);
+      this.treeBox.rowCountChanged(idx + 1, -deletecount);
+    }
+    else {
+      // add this window's tab rows to the view
+      var toinsert = gTreeData[idx].tabs;
+      for (var i = 0; i < toinsert.length; i++)
+        gTreeData.splice(idx + i + 1, 0, toinsert[i]);
+      this.treeBox.rowCountChanged(idx + 1, toinsert.length);
+    }
+    item.open = !item.open;
+    this.treeBox.invalidateRow(idx);
+  },
+
+  getCellProperties: function(idx, column) {
+    if (column.id == "restore" && this.isContainer(idx) && gTreeData[idx].checked === 0)
+      return "partial";
+    if (column.id == "title")
+      return this.getImageSrc(idx, column) ? "icon" : "noicon";
+
+    return "";
+  },
+
+  getRowProperties: function(idx) {
+    var winState = gTreeData[idx].parent || gTreeData[idx];
+    if (winState.ix % 2 != 0)
+      return "alternate";
+
+    return "";
+  },
+
+  getImageSrc: function(idx, column) {
+    if (column.id == "title")
+      return gTreeData[idx].src || null;
+    return null;
+  },
+
+  getProgressMode : function(idx, column) { },
+  cycleHeader: function(column) { },
+  cycleCell: function(idx, column) { },
+  selectionChanged: function() { },
+  performAction: function(action) { },
+  performActionOnCell: function(action, index, column) { },
+  getColumnProperties: function(column) { return ""; }
+};
diff --git a/application/basilisk/components/sessionstore/content/aboutSessionRestore.xhtml b/application/basilisk/components/sessionstore/content/aboutSessionRestore.xhtml
new file mode 100644
index 0000000000..bcd9084e77
--- /dev/null
+++ b/application/basilisk/components/sessionstore/content/aboutSessionRestore.xhtml
@@ -0,0 +1,86 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<!--
+# This Source Code Form is subject to the terms of the Mozilla Public
+# License, v. 2.0. If a copy of the MPL was not distributed with this
+# file, You can obtain one at http://mozilla.org/MPL/2.0/.
+-->
+<!DOCTYPE html [
+  <!ENTITY % htmlDTD PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "DTD/xhtml1-strict.dtd">
+  %htmlDTD;
+  <!ENTITY % netErrorDTD SYSTEM "chrome://global/locale/netError.dtd">
+  %netErrorDTD;
+  <!ENTITY % globalDTD SYSTEM "chrome://global/locale/global.dtd">
+  %globalDTD;
+  <!ENTITY % restorepageDTD SYSTEM "chrome://browser/locale/aboutSessionRestore.dtd">
+  %restorepageDTD;
+]>
+
+<html xmlns="http://www.w3.org/1999/xhtml" xmlns:xul="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul">
+  <head>
+    <title>&restorepage.tabtitle;</title>
+    <link rel="stylesheet" href="chrome://global/skin/in-content/info-pages.css" type="text/css" media="all"/>
+    <link rel="stylesheet" href="chrome://browser/skin/aboutSessionRestore.css" type="text/css" media="all"/>
+    <link rel="icon" type="image/png" href="chrome://global/skin/icons/warning-16.png"/>
+
+    <script type="application/javascript;version=1.8" src="chrome://browser/content/aboutSessionRestore.js"/>
+  </head>
+
+  <body dir="&locale.dir;">
+
+    <div class="container restore-chosen">
+
+      <div class="title">
+        <h1 class="title-text">&restorepage.errorTitle;</h1>
+      </div>
+      <div class="description">
+        <p>&restorepage.problemDesc;</p>
+
+        <div id="errorLongDesc">
+          <p>&restorepage.tryThis;</p>
+          <ul>
+            <li>&restorepage.restoreSome;</li>
+            <li>&restorepage.startNew;</li>
+          </ul>
+        </div>
+      </div>
+      <div class="tree-container" available="true">
+        <xul:tree id="tabList" seltype="single" hidecolumnpicker="true"
+              onclick="onListClick(event);" onkeydown="onListKeyDown(event);"
+              _window_label="&restorepage.windowLabel;">
+          <xul:treecols>
+            <xul:treecol cycler="true" id="restore" type="checkbox" label="&restorepage.restoreHeader;"/>
+            <xul:splitter class="tree-splitter"/>
+            <xul:treecol primary="true" id="title" label="&restorepage.listHeader;" flex="1"/>
+          </xul:treecols>
+          <xul:treechildren flex="1"/>
+        </xul:tree>
+      </div>
+      <div class="button-container">
+#ifdef XP_UNIX
+        <xul:button id="errorCancel"
+                label="&restorepage.closeButton;"
+                accesskey="&restorepage.close.access;"
+                oncommand="startNewSession();"/>
+        <xul:button class="primary"
+                id="errorTryAgain"
+                label="&restorepage.tryagainButton;"
+                accesskey="&restorepage.restore.access;"
+                oncommand="restoreSession();"/>
+#else
+        <xul:button class="primary"
+                id="errorTryAgain"
+                label="&restorepage.tryagainButton;"
+                accesskey="&restorepage.restore.access;"
+                oncommand="restoreSession();"/>
+        <xul:button id="errorCancel"
+                label="&restorepage.closeButton;"
+                accesskey="&restorepage.close.access;"
+                oncommand="startNewSession();"/>
+#endif
+      </div>
+      <!-- holds the session data for when the tab is closed -->
+      <input type="text" id="sessionData" style="display: none;"/>
+    </div>
+
+  </body>
+</html>
diff --git a/application/basilisk/components/sessionstore/content/content-sessionStore.js b/application/basilisk/components/sessionstore/content/content-sessionStore.js
new file mode 100644
index 0000000000..858e35750e
--- /dev/null
+++ b/application/basilisk/components/sessionstore/content/content-sessionStore.js
@@ -0,0 +1,897 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+"use strict";
+
+function debug(msg) {
+  Services.console.logStringMessage("SessionStoreContent: " + msg);
+}
+
+var Cu = Components.utils;
+var Cc = Components.classes;
+var Ci = Components.interfaces;
+var Cr = Components.results;
+
+Cu.import("resource://gre/modules/XPCOMUtils.jsm", this);
+Cu.import("resource://gre/modules/Timer.jsm", this);
+
+XPCOMUtils.defineLazyModuleGetter(this, "FormData",
+  "resource://gre/modules/FormData.jsm");
+XPCOMUtils.defineLazyModuleGetter(this, "Preferences",
+  "resource://gre/modules/Preferences.jsm");
+
+XPCOMUtils.defineLazyModuleGetter(this, "DocShellCapabilities",
+  "resource:///modules/sessionstore/DocShellCapabilities.jsm");
+XPCOMUtils.defineLazyModuleGetter(this, "PageStyle",
+  "resource:///modules/sessionstore/PageStyle.jsm");
+XPCOMUtils.defineLazyModuleGetter(this, "ScrollPosition",
+  "resource://gre/modules/ScrollPosition.jsm");
+XPCOMUtils.defineLazyModuleGetter(this, "SessionHistory",
+  "resource:///modules/sessionstore/SessionHistory.jsm");
+XPCOMUtils.defineLazyModuleGetter(this, "SessionStorage",
+  "resource:///modules/sessionstore/SessionStorage.jsm");
+
+Cu.import("resource:///modules/sessionstore/FrameTree.jsm", this);
+var gFrameTree = new FrameTree(this);
+
+Cu.import("resource:///modules/sessionstore/ContentRestore.jsm", this);
+XPCOMUtils.defineLazyGetter(this, 'gContentRestore',
+                            () => { return new ContentRestore(this) });
+
+// The current epoch.
+var gCurrentEpoch = 0;
+
+// A bound to the size of data to store for DOM Storage.
+const DOM_STORAGE_MAX_CHARS = 10000000; // 10M characters
+
+// This pref controls whether or not we send updates to the parent on a timeout
+// or not, and should only be used for tests or debugging.
+const TIMEOUT_DISABLED_PREF = "browser.sessionstore.debug.no_auto_updates";
+
+const kNoIndex = Number.MAX_SAFE_INTEGER;
+const kLastIndex = Number.MAX_SAFE_INTEGER - 1;
+
+/**
+ * Returns a lazy function that will evaluate the given
+ * function |fn| only once and cache its return value.
+ */
+function createLazy(fn) {
+  let cached = false;
+  let cachedValue = null;
+
+  return function lazy() {
+    if (!cached) {
+      cachedValue = fn();
+      cached = true;
+    }
+
+    return cachedValue;
+  };
+}
+
+/**
+ * Listens for and handles content events that we need for the
+ * session store service to be notified of state changes in content.
+ */
+var EventListener = {
+
+  init: function () {
+    addEventListener("load", this, true);
+  },
+
+  handleEvent: function (event) {
+    // Ignore load events from subframes.
+    if (event.target != content.document) {
+      return;
+    }
+
+    if (content.document.documentURI.startsWith("about:reader")) {
+      if (event.type == "load" &&
+          !content.document.body.classList.contains("loaded")) {
+        // Don't restore the scroll position of an about:reader page at this
+        // point; listen for the custom event dispatched from AboutReader.jsm.
+        content.addEventListener("AboutReaderContentReady", this);
+        return;
+      }
+
+      content.removeEventListener("AboutReaderContentReady", this);
+    }
+
+    // Restore the form data and scroll position. If we're not currently
+    // restoring a tab state then this call will simply be a noop.
+    gContentRestore.restoreDocument();
+  }
+};
+
+/**
+ * Listens for and handles messages sent by the session store service.
+ */
+var MessageListener = {
+
+  MESSAGES: [
+    "SessionStore:restoreHistory",
+    "SessionStore:restoreTabContent",
+    "SessionStore:resetRestore",
+    "SessionStore:flush",
+  ],
+
+  init: function () {
+    this.MESSAGES.forEach(m => addMessageListener(m, this));
+  },
+
+  receiveMessage: function ({name, data}) {
+    // The docShell might be gone. Don't process messages,
+    // that will just lead to errors anyway.
+    if (!docShell) {
+      return;
+    }
+
+    // A fresh tab always starts with epoch=0. The parent has the ability to
+    // override that to signal a new era in this tab's life. This enables it
+    // to ignore async messages that were already sent but not yet received
+    // and would otherwise confuse the internal tab state.
+    if (data.epoch && data.epoch != gCurrentEpoch) {
+      gCurrentEpoch = data.epoch;
+    }
+
+    switch (name) {
+      case "SessionStore:restoreHistory":
+        this.restoreHistory(data);
+        break;
+      case "SessionStore:restoreTabContent":
+        this.restoreTabContent(data);
+        break;
+      case "SessionStore:resetRestore":
+        gContentRestore.resetRestore();
+        break;
+      case "SessionStore:flush":
+        this.flush(data);
+        break;
+      default:
+        debug("received unknown message '" + name + "'");
+        break;
+    }
+  },
+
+  restoreHistory({epoch, tabData, loadArguments, isRemotenessUpdate}) {
+    gContentRestore.restoreHistory(tabData, loadArguments, {
+      // Note: The callbacks passed here will only be used when a load starts
+      // that was not initiated by sessionstore itself. This can happen when
+      // some code calls browser.loadURI() or browser.reload() on a pending
+      // browser/tab.
+
+      onLoadStarted() {
+        // Notify the parent that the tab is no longer pending.
+        sendSyncMessage("SessionStore:restoreTabContentStarted", {epoch});
+      },
+
+      onLoadFinished() {
+        // Tell SessionStore.jsm that it may want to restore some more tabs,
+        // since it restores a max of MAX_CONCURRENT_TAB_RESTORES at a time.
+        sendAsyncMessage("SessionStore:restoreTabContentComplete", {epoch});
+      }
+    });
+
+    // When restoreHistory finishes, we send a synchronous message to
+    // SessionStore.jsm so that it can run SSTabRestoring. Users of
+    // SSTabRestoring seem to get confused if chrome and content are out of
+    // sync about the state of the restore (particularly regarding
+    // docShell.currentURI). Using a synchronous message is the easiest way
+    // to temporarily synchronize them.
+    sendSyncMessage("SessionStore:restoreHistoryComplete", {epoch, isRemotenessUpdate});
+  },
+
+  restoreTabContent({loadArguments, isRemotenessUpdate}) {
+    let epoch = gCurrentEpoch;
+
+    // We need to pass the value of didStartLoad back to SessionStore.jsm.
+    let didStartLoad = gContentRestore.restoreTabContent(loadArguments, isRemotenessUpdate, () => {
+      // Tell SessionStore.jsm that it may want to restore some more tabs,
+      // since it restores a max of MAX_CONCURRENT_TAB_RESTORES at a time.
+      sendAsyncMessage("SessionStore:restoreTabContentComplete", {epoch, isRemotenessUpdate});
+    });
+
+    sendAsyncMessage("SessionStore:restoreTabContentStarted", {epoch, isRemotenessUpdate});
+
+    if (!didStartLoad) {
+      // Pretend that the load succeeded so that event handlers fire correctly.
+      sendAsyncMessage("SessionStore:restoreTabContentComplete", {epoch, isRemotenessUpdate});
+    }
+  },
+
+  flush({id}) {
+    // Flush the message queue, send the latest updates.
+    MessageQueue.send({flushID: id});
+  }
+};
+
+/**
+ * Listens for changes to the session history. Whenever the user navigates
+ * we will collect URLs and everything belonging to session history.
+ *
+ * Causes a SessionStore:update message to be sent that contains the current
+ * session history.
+ *
+ * Example:
+ *   {entries: [{url: "about:mozilla", ...}, ...], index: 1}
+ */
+var SessionHistoryListener = {
+  init: function () {
+    // The frame tree observer is needed to handle initial subframe loads.
+    // It will redundantly invalidate with the SHistoryListener in some cases
+    // but these invalidations are very cheap.
+    gFrameTree.addObserver(this);
+
+    // By adding the SHistoryListener immediately, we will unfortunately be
+    // notified of every history entry as the tab is restored. We don't bother
+    // waiting to add the listener later because these notifications are cheap.
+    // We will likely only collect once since we are batching collection on
+    // a delay.
+    docShell.QueryInterface(Ci.nsIWebNavigation).sessionHistory.
+      addSHistoryListener(this);
+
+    // Collect data if we start with a non-empty shistory.
+    if (!SessionHistory.isEmpty(docShell)) {
+      this.collect();
+      // When a tab is detached from the window, for the new window there is a
+      // new SessionHistoryListener created. Normally it is empty at this point
+      // but in a test env. the initial about:blank might have a children in which
+      // case we fire off a history message here with about:blank in it. If we
+      // don't do it ASAP then there is going to be a browser swap and the parent
+      // will be all confused by that message.
+      MessageQueue.send();
+    }
+
+    // Listen for page title changes.
+    addEventListener("DOMTitleChanged", this);
+  },
+
+  uninit: function () {
+    let sessionHistory = docShell.QueryInterface(Ci.nsIWebNavigation).sessionHistory;
+    if (sessionHistory) {
+      sessionHistory.removeSHistoryListener(this);
+    }
+  },
+
+  collect: function () {
+    this._fromIdx = kNoIndex;
+    if (docShell) {
+      MessageQueue.push("history", () => SessionHistory.collect(docShell));
+    }
+  },
+
+  _fromIdx: kNoIndex,
+
+  // History can grow relatively big with the nested elements, so if we don't have to, we
+  // don't want to send the entire history all the time. For a simple optimization
+  // we keep track of the smallest index from after any change has occured and we just send
+  // the elements from that index. If something more complicated happens we just clear it
+  // and send the entire history. We always send the additional info like the current selected
+  // index (so for going back and forth between history entries we set the index to kLastIndex
+  // if nothing else changed send an empty array and the additonal info like the selected index)
+  collectFrom: function (idx) {
+    if (this._fromIdx <= idx) {
+      // If we already know that we need to update history fromn index N we can ignore any changes
+      // tha happened with an element with index larger than N.
+      // Note: initially we use kNoIndex which is MAX_SAFE_INTEGER which means we don't ignore anything
+      // here, and in case of navigation in the history back and forth we use kLastIndex which ignores
+      // only the subsequent navigations, but not any new elements added.
+      return;
+    }
+
+    this._fromIdx = idx;
+    MessageQueue.push("historychange", () => {
+      if (this._fromIdx === kNoIndex) {
+        return null;
+      }
+
+      let history = SessionHistory.collect(docShell);
+      if (kLastIndex == idx) {
+        history.entries = [];
+      } else {
+        history.entries.splice(0, this._fromIdx + 1);
+      }
+
+      history.fromIdx = this._fromIdx;
+
+      this._fromIdx = kNoIndex;
+      return history;
+    });
+  },
+
+  handleEvent(event) {
+    this.collect();
+  },
+
+  onFrameTreeCollected: function () {
+    this.collect();
+  },
+
+  onFrameTreeReset: function () {
+    this.collect();
+  },
+
+  OnHistoryNewEntry: function (newURI, oldIndex) {
+    this.collectFrom(oldIndex);
+  },
+
+  OnHistoryGoBack: function (backURI) {
+    this.collectFrom(kLastIndex);
+    return true;
+  },
+
+  OnHistoryGoForward: function (forwardURI) {
+    this.collectFrom(kLastIndex);
+    return true;
+  },
+
+  OnHistoryGotoIndex: function (index, gotoURI) {
+    this.collectFrom(kLastIndex);
+    return true;
+  },
+
+  OnHistoryPurge: function (numEntries) {
+    this.collect();
+    return true;
+  },
+
+  OnHistoryReload: function (reloadURI, reloadFlags) {
+    this.collect();
+    return true;
+  },
+
+  OnHistoryReplaceEntry: function (index) {
+    this.collect();
+  },
+
+  QueryInterface: XPCOMUtils.generateQI([
+    Ci.nsISHistoryListener,
+    Ci.nsISupportsWeakReference
+  ])
+};
+
+/**
+ * Listens for scroll position changes. Whenever the user scrolls the top-most
+ * frame we update the scroll position and will restore it when requested.
+ *
+ * Causes a SessionStore:update message to be sent that contains the current
+ * scroll positions as a tree of strings. If no frame of the whole frame tree
+ * is scrolled this will return null so that we don't tack a property onto
+ * the tabData object in the parent process.
+ *
+ * Example:
+ *   {scroll: "100,100", children: [null, null, {scroll: "200,200"}]}
+ */
+var ScrollPositionListener = {
+  init: function () {
+    addEventListener("scroll", this);
+    gFrameTree.addObserver(this);
+  },
+
+  handleEvent: function (event) {
+    let frame = event.target.defaultView;
+
+    // Don't collect scroll data for frames created at or after the load event
+    // as SessionStore can't restore scroll data for those.
+    if (gFrameTree.contains(frame)) {
+      MessageQueue.push("scroll", () => this.collect());
+    }
+  },
+
+  onFrameTreeCollected: function () {
+    MessageQueue.push("scroll", () => this.collect());
+  },
+
+  onFrameTreeReset: function () {
+    MessageQueue.push("scroll", () => null);
+  },
+
+  collect: function () {
+    return gFrameTree.map(ScrollPosition.collect);
+  }
+};
+
+/**
+ * Listens for changes to input elements. Whenever the value of an input
+ * element changes we will re-collect data for the current frame tree and send
+ * a message to the parent process.
+ *
+ * Causes a SessionStore:update message to be sent that contains the form data
+ * for all reachable frames.
+ *
+ * Example:
+ *   {
+ *     formdata: {url: "http://mozilla.org/", id: {input_id: "input value"}},
+ *     children: [
+ *       null,
+ *       {url: "http://sub.mozilla.org/", id: {input_id: "input value 2"}}
+ *     ]
+ *   }
+ */
+var FormDataListener = {
+  init: function () {
+    addEventListener("input", this, true);
+    addEventListener("change", this, true);
+    gFrameTree.addObserver(this);
+  },
+
+  handleEvent: function (event) {
+    let frame = event.target.ownerGlobal;
+
+    // Don't collect form data for frames created at or after the load event
+    // as SessionStore can't restore form data for those.
+    if (gFrameTree.contains(frame)) {
+      MessageQueue.push("formdata", () => this.collect());
+    }
+  },
+
+  onFrameTreeReset: function () {
+    MessageQueue.push("formdata", () => null);
+  },
+
+  collect: function () {
+    return gFrameTree.map(FormData.collect);
+  }
+};
+
+/**
+ * Listens for changes to the page style. Whenever a different page style is
+ * selected or author styles are enabled/disabled we send a message with the
+ * currently applied style to the chrome process.
+ *
+ * Causes a SessionStore:update message to be sent that contains the currently
+ * selected pageStyle for all reachable frames.
+ *
+ * Example:
+ *   {pageStyle: "Dusk", children: [null, {pageStyle: "Mozilla"}]}
+ */
+var PageStyleListener = {
+  init: function () {
+    Services.obs.addObserver(this, "author-style-disabled-changed", false);
+    Services.obs.addObserver(this, "style-sheet-applicable-state-changed", false);
+    gFrameTree.addObserver(this);
+  },
+
+  uninit: function () {
+    Services.obs.removeObserver(this, "author-style-disabled-changed");
+    Services.obs.removeObserver(this, "style-sheet-applicable-state-changed");
+  },
+
+  observe: function (subject, topic) {
+    let frame = subject.defaultView;
+
+    if (frame && gFrameTree.contains(frame)) {
+      MessageQueue.push("pageStyle", () => this.collect());
+    }
+  },
+
+  collect: function () {
+    return PageStyle.collect(docShell, gFrameTree);
+  },
+
+  onFrameTreeCollected: function () {
+    MessageQueue.push("pageStyle", () => this.collect());
+  },
+
+  onFrameTreeReset: function () {
+    MessageQueue.push("pageStyle", () => null);
+  }
+};
+
+/**
+ * Listens for changes to docShell capabilities. Whenever a new load is started
+ * we need to re-check the list of capabilities and send message when it has
+ * changed.
+ *
+ * Causes a SessionStore:update message to be sent that contains the currently
+ * disabled docShell capabilities (all nsIDocShell.allow* properties set to
+ * false) as a string - i.e. capability names separate by commas.
+ */
+var DocShellCapabilitiesListener = {
+  /**
+   * This field is used to compare the last docShell capabilities to the ones
+   * that have just been collected. If nothing changed we won't send a message.
+   */
+  _latestCapabilities: "",
+
+  init: function () {
+    gFrameTree.addObserver(this);
+  },
+
+  /**
+   * onFrameTreeReset() is called as soon as we start loading a page.
+   */
+  onFrameTreeReset: function() {
+    // The order of docShell capabilities cannot change while we're running
+    // so calling join() without sorting before is totally sufficient.
+    let caps = DocShellCapabilities.collect(docShell).join(",");
+
+    // Send new data only when the capability list changes.
+    if (caps != this._latestCapabilities) {
+      this._latestCapabilities = caps;
+      MessageQueue.push("disallow", () => caps || null);
+    }
+  }
+};
+
+/**
+ * Listens for changes to the DOMSessionStorage. Whenever new keys are added,
+ * existing ones removed or changed, or the storage is cleared we will send a
+ * message to the parent process containing up-to-date sessionStorage data.
+ *
+ * Causes a SessionStore:update message to be sent that contains the current
+ * DOMSessionStorage contents. The data is a nested object using host names
+ * as keys and per-host DOMSessionStorage data as values.
+ */
+var SessionStorageListener = {
+  init: function () {
+    addEventListener("MozSessionStorageChanged", this, true);
+    Services.obs.addObserver(this, "browser:purge-domain-data", false);
+    gFrameTree.addObserver(this);
+  },
+
+  uninit: function () {
+    Services.obs.removeObserver(this, "browser:purge-domain-data");
+  },
+
+  handleEvent: function (event) {
+    if (gFrameTree.contains(event.target)) {
+      this.collectFromEvent(event);
+    }
+  },
+
+  observe: function () {
+    // Collect data on the next tick so that any other observer
+    // that needs to purge data can do its work first.
+    setTimeout(() => this.collect(), 0);
+  },
+
+  // Before DOM Storage can be written to disk, it needs to be serialized
+  // for sending across frames/processes, then again to be sent across
+  // threads, then again to be put in a buffer for the disk. Each of these
+  // serializations is an opportunity to OOM and (depending on the site of
+  // the OOM), either crash, lose all data for the frame or lose all data
+  // for the application.
+  //
+  // In order to avoid this, compute an estimate of the size of the
+  // object, and block SessionStorage items that are too large. As
+  // we also don't want to cause an OOM here, we use a quick and memory-
+  // efficient approximation: we compute the total sum of string lengths
+  // involved in this object.
+  estimateStorageSize: function(collected) {
+    if (!collected) {
+      return 0;
+    }
+
+    let size = 0;
+    for (let host of Object.keys(collected)) {
+      size += host.length;
+      let perHost = collected[host];
+      for (let key of Object.keys(perHost)) {
+        size += key.length;
+        let perKey = perHost[key];
+        size += perKey.length;
+      }
+    }
+
+    return size;
+  },
+
+  // We don't want to send all the session storage data for all the frames
+  // for every change. So if only a few value changed we send them over as
+  // a "storagechange" event. If however for some reason before we send these
+  // changes we have to send over the entire sessions storage data, we just
+  // reset these changes.
+  _changes: undefined,
+
+  resetChanges: function () {
+    this._changes = undefined;
+  },
+
+  collectFromEvent: function (event) {
+    // TODO: we should take browser.sessionstore.dom_storage_limit into an account here.
+    if (docShell) {
+      let {url, key, newValue} = event;
+      let uri = Services.io.newURI(url, null, null);
+      let domain = uri.prePath;
+      if (!this._changes) {
+        this._changes = {};
+      }
+      if (!this._changes[domain]) {
+        this._changes[domain] = {};
+      }
+      this._changes[domain][key] = newValue;
+
+      MessageQueue.push("storagechange", () => {
+        let tmp = this._changes;
+        // If there were multiple changes we send them merged.
+        // First one will collect all the changes the rest of
+        // these messages will be ignored.
+        this.resetChanges();
+        return tmp;
+      });
+    }
+  },
+
+  collect: function () {
+    if (docShell) {
+      // We need the entire session storage, let's reset the pending individual change
+      // messages.
+      this.resetChanges();
+      MessageQueue.push("storage", () => {
+        let collected = SessionStorage.collect(docShell, gFrameTree);
+
+        if (collected == null) {
+          return collected;
+        }
+
+        let size = this.estimateStorageSize(collected);
+
+        MessageQueue.push("telemetry", () => ({ FX_SESSION_RESTORE_DOM_STORAGE_SIZE_ESTIMATE_CHARS: size }));
+        if (size > Preferences.get("browser.sessionstore.dom_storage_limit", DOM_STORAGE_MAX_CHARS)) {
+          // Rather than keeping the old storage, which wouldn't match the rest
+          // of the state of the page, empty the storage. DOM storage will be
+          // recollected the next time and stored if it is now small enough.
+          return {};
+        }
+
+        return collected;
+      });
+    }
+  },
+
+  onFrameTreeCollected: function () {
+    this.collect();
+  },
+
+  onFrameTreeReset: function () {
+    this.collect();
+  }
+};
+
+/**
+ * Listen for changes to the privacy status of the tab.
+ * By definition, tabs start in non-private mode.
+ *
+ * Causes a SessionStore:update message to be sent for
+ * field "isPrivate". This message contains
+ *  |true| if the tab is now private
+ *  |null| if the tab is now public - the field is therefore
+ *  not saved.
+ */
+var PrivacyListener = {
+  init: function() {
+    docShell.addWeakPrivacyTransitionObserver(this);
+
+    // Check that value at startup as it might have
+    // been set before the frame script was loaded.
+    if (docShell.QueryInterface(Ci.nsILoadContext).usePrivateBrowsing) {
+      MessageQueue.push("isPrivate", () => true);
+    }
+  },
+
+  // Ci.nsIPrivacyTransitionObserver
+  privateModeChanged: function(enabled) {
+    MessageQueue.push("isPrivate", () => enabled || null);
+  },
+
+  QueryInterface: XPCOMUtils.generateQI([Ci.nsIPrivacyTransitionObserver,
+                                         Ci.nsISupportsWeakReference])
+};
+
+/**
+ * A message queue that takes collected data and will take care of sending it
+ * to the chrome process. It allows flushing using synchronous messages and
+ * takes care of any race conditions that might occur because of that. Changes
+ * will be batched if they're pushed in quick succession to avoid a message
+ * flood.
+ */
+var MessageQueue = {
+  /**
+   * A map (string -> lazy fn) holding lazy closures of all queued data
+   * collection routines. These functions will return data collected from the
+   * docShell.
+   */
+  _data: new Map(),
+
+  /**
+   * The delay (in ms) used to delay sending changes after data has been
+   * invalidated.
+   */
+  BATCH_DELAY_MS: 1000,
+
+  /**
+   * The current timeout ID, null if there is no queue data. We use timeouts
+   * to damp a flood of data changes and send lots of changes as one batch.
+   */
+  _timeout: null,
+
+  /**
+   * Whether or not sending batched messages on a timer is disabled. This should
+   * only be used for debugging or testing. If you need to access this value,
+   * you should probably use the timeoutDisabled getter.
+   */
+  _timeoutDisabled: false,
+
+  /**
+   * True if batched messages are not being fired on a timer. This should only
+   * ever be true when debugging or during tests.
+   */
+  get timeoutDisabled() {
+    return this._timeoutDisabled;
+  },
+
+  /**
+   * Disables sending batched messages on a timer. Also cancels any pending
+   * timers.
+   */
+  set timeoutDisabled(val) {
+    this._timeoutDisabled = val;
+
+    if (val && this._timeout) {
+      clearTimeout(this._timeout);
+      this._timeout = null;
+    }
+
+    return val;
+  },
+
+  init() {
+    this.timeoutDisabled =
+      Services.prefs.getBoolPref(TIMEOUT_DISABLED_PREF);
+
+    Services.prefs.addObserver(TIMEOUT_DISABLED_PREF, this, false);
+  },
+
+  uninit() {
+    Services.prefs.removeObserver(TIMEOUT_DISABLED_PREF, this);
+  },
+
+  observe(subject, topic, data) {
+    if (topic == "nsPref:changed" && data == TIMEOUT_DISABLED_PREF) {
+      this.timeoutDisabled =
+        Services.prefs.getBoolPref(TIMEOUT_DISABLED_PREF);
+    }
+  },
+
+  /**
+   * Pushes a given |value| onto the queue. The given |key| represents the type
+   * of data that is stored and can override data that has been queued before
+   * but has not been sent to the parent process, yet.
+   *
+   * @param key (string)
+   *        A unique identifier specific to the type of data this is passed.
+   * @param fn (function)
+   *        A function that returns the value that will be sent to the parent
+   *        process.
+   */
+  push: function (key, fn) {
+    this._data.set(key, createLazy(fn));
+
+    if (!this._timeout && !this._timeoutDisabled) {
+      // Wait a little before sending the message to batch multiple changes.
+      this._timeout = setTimeout(() => this.send(), this.BATCH_DELAY_MS);
+    }
+  },
+
+  /**
+   * Sends queued data to the chrome process.
+   *
+   * @param options (object)
+   *        {flushID: 123} to specify that this is a flush
+   *        {isFinal: true} to signal this is the final message sent on unload
+   */
+  send: function (options = {}) {
+    // Looks like we have been called off a timeout after the tab has been
+    // closed. The docShell is gone now and we can just return here as there
+    // is nothing to do.
+    if (!docShell) {
+      return;
+    }
+
+    if (this._timeout) {
+      clearTimeout(this._timeout);
+      this._timeout = null;
+    }
+
+    let flushID = (options && options.flushID) || 0;
+
+    let durationMs = Date.now();
+
+    let data = {};
+    let telemetry = {};
+    for (let [key, func] of this._data) {
+      let value = func();
+      if (key == "telemetry") {
+        for (let histogramId of Object.keys(value)) {
+          telemetry[histogramId] = value[histogramId];
+        }
+      } else if (value || (key != "storagechange" && key != "historychange")) {
+        data[key] = value;
+      }
+    }
+
+    this._data.clear();
+
+    durationMs = Date.now() - durationMs;
+    telemetry.FX_SESSION_RESTORE_CONTENT_COLLECT_DATA_LONGEST_OP_MS = durationMs;
+
+    try {
+      // Send all data to the parent process.
+      sendAsyncMessage("SessionStore:update", {
+        data, telemetry, flushID,
+        isFinal: options.isFinal || false,
+        epoch: gCurrentEpoch
+      });
+    } catch (ex if ex && ex.result == Cr.NS_ERROR_OUT_OF_MEMORY) {
+      let telemetry = {
+        FX_SESSION_RESTORE_SEND_UPDATE_CAUSED_OOM: 1
+      };
+      sendAsyncMessage("SessionStore:error", {
+        telemetry
+      });
+    }
+  },
+};
+
+EventListener.init();
+MessageListener.init();
+FormDataListener.init();
+PageStyleListener.init();
+SessionHistoryListener.init();
+SessionStorageListener.init();
+ScrollPositionListener.init();
+DocShellCapabilitiesListener.init();
+PrivacyListener.init();
+MessageQueue.init();
+
+function handleRevivedTab() {
+  if (!content) {
+    removeEventListener("pagehide", handleRevivedTab);
+    return;
+  }
+
+  if (content.document.documentURI.startsWith("about:tabcrashed")) {
+    if (Services.appinfo.processType != Services.appinfo.PROCESS_TYPE_DEFAULT) {
+      // Sanity check - we'd better be loading this in a non-remote browser.
+      throw new Error("We seem to be navigating away from about:tabcrashed in " +
+                      "a non-remote browser. This should really never happen.");
+    }
+
+    removeEventListener("pagehide", handleRevivedTab);
+
+    // Notify the parent.
+    sendAsyncMessage("SessionStore:crashedTabRevived");
+  }
+}
+
+// If we're browsing from the tab crashed UI to a blacklisted URI that keeps
+// this browser non-remote, we'll handle that in a pagehide event.
+addEventListener("pagehide", handleRevivedTab);
+
+addEventListener("unload", () => {
+  // Upon frameLoader destruction, send a final update message to
+  // the parent and flush all data currently held in the child.
+  MessageQueue.send({isFinal: true});
+
+  // If we're browsing from the tab crashed UI to a URI that causes the tab
+  // to go remote again, we catch this in the unload event handler, because
+  // swapping out the non-remote browser for a remote one in
+  // tabbrowser.xml's updateBrowserRemoteness doesn't cause the pagehide
+  // event to be fired.
+  handleRevivedTab();
+
+  // Remove all registered nsIObservers.
+  PageStyleListener.uninit();
+  SessionStorageListener.uninit();
+  SessionHistoryListener.uninit();
+  MessageQueue.uninit();
+
+  // Remove progress listeners.
+  gContentRestore.resetRestore();
+
+  // We don't need to take care of any gFrameTree observers as the gFrameTree
+  // will die with the content script. The same goes for the privacy transition
+  // observer that will die with the docShell when the tab is closed.
+});
diff --git a/application/basilisk/components/sessionstore/jar.mn b/application/basilisk/components/sessionstore/jar.mn
new file mode 100644
index 0000000000..7e5bc07dc6
--- /dev/null
+++ b/application/basilisk/components/sessionstore/jar.mn
@@ -0,0 +1,8 @@
+# This Source Code Form is subject to the terms of the Mozilla Public
+# License, v. 2.0. If a copy of the MPL was not distributed with this
+# file, You can obtain one at http://mozilla.org/MPL/2.0/.
+
+browser.jar:
+*   content/browser/aboutSessionRestore.xhtml             (content/aboutSessionRestore.xhtml)
+    content/browser/aboutSessionRestore.js                (content/aboutSessionRestore.js)
+    content/browser/content-sessionStore.js               (content/content-sessionStore.js)
diff --git a/application/basilisk/components/sessionstore/moz.build b/application/basilisk/components/sessionstore/moz.build
new file mode 100644
index 0000000000..3117f02c7e
--- /dev/null
+++ b/application/basilisk/components/sessionstore/moz.build
@@ -0,0 +1,46 @@
+# -*- Mode: python; indent-tabs-mode: nil; tab-width: 40 -*-
+# vim: set filetype=python:
+# This Source Code Form is subject to the terms of the Mozilla Public
+# License, v. 2.0. If a copy of the MPL was not distributed with this
+# file, You can obtain one at http://mozilla.org/MPL/2.0/.
+
+JAR_MANIFESTS += ['jar.mn']
+
+XPIDL_SOURCES += [
+    'nsISessionStartup.idl',
+    'nsISessionStore.idl',
+]
+
+XPIDL_MODULE = 'sessionstore'
+
+EXTRA_COMPONENTS += [
+    'nsSessionStartup.js',
+    'nsSessionStore.js',
+    'nsSessionStore.manifest',
+]
+
+EXTRA_JS_MODULES.sessionstore = [
+    'ContentRestore.jsm',
+    'DocShellCapabilities.jsm',
+    'FrameTree.jsm',
+    'GlobalState.jsm',
+    'PageStyle.jsm',
+    'PrivacyFilter.jsm',
+    'PrivacyLevel.jsm',
+    'RecentlyClosedTabsAndWindowsMenuUtils.jsm',
+    'RunState.jsm',
+    'SessionCookies.jsm',
+    'SessionFile.jsm',
+    'SessionHistory.jsm',
+    'SessionMigration.jsm',
+    'SessionSaver.jsm',
+    'SessionStorage.jsm',
+    'SessionStore.jsm',
+    'SessionWorker.js',
+    'SessionWorker.jsm',
+    'StartupPerformance.jsm',
+    'TabAttributes.jsm',
+    'TabState.jsm',
+    'TabStateCache.jsm',
+    'TabStateFlusher.jsm',
+]
diff --git a/application/basilisk/components/sessionstore/nsISessionStartup.idl b/application/basilisk/components/sessionstore/nsISessionStartup.idl
new file mode 100644
index 0000000000..2321ac3103
--- /dev/null
+++ b/application/basilisk/components/sessionstore/nsISessionStartup.idl
@@ -0,0 +1,66 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "nsISupports.idl"
+
+/**
+ * nsISessionStore keeps track of the current browsing state - i.e.
+ * tab history, cookies, scroll state, form data, and window features
+ * - and allows to restore everything into one window.
+ */
+
+[scriptable, uuid(934697e4-3807-47f8-b6c9-6caa8d83ccd1)]
+interface nsISessionStartup: nsISupports
+{
+  /**
+   * Return a promise that is resolved once initialization
+   * is complete.
+   */
+  readonly attribute jsval onceInitialized;
+
+  // Get session state
+  readonly attribute jsval state;
+
+  /**
+   * Determines whether there is a pending session restore. Should only be
+   * called after initialization has completed.
+   */
+  boolean doRestore();
+
+  /**
+   * Determines whether automatic session restoration is enabled for this
+   * launch of the browser. This does not include crash restoration, and will
+   * return false if restoration will only be caused by a crash.
+   */
+  boolean isAutomaticRestoreEnabled();
+
+  /**
+   * Returns whether we will restore a session that ends up replacing the
+   * homepage. The browser uses this to not start loading the homepage if
+   * we're going to stop its load anyway shortly after.
+   *
+   * This is meant to be an optimization for the average case that loading the
+   * session file finishes before we may want to start loading the default
+   * homepage. Should this be called before the session file has been read it
+   * will just return false.
+   */
+  readonly attribute bool willOverrideHomepage;
+
+  /**
+   * What type of session we're restoring.
+   * NO_SESSION       There is no data available from the previous session
+   * RECOVER_SESSION  The last session crashed. It will either be restored or
+   *                  about:sessionrestore will be shown.
+   * RESUME_SESSION   The previous session should be restored at startup
+   * DEFER_SESSION    The previous session is fine, but it shouldn't be restored
+   *                  without explicit action (with the exception of pinned tabs)
+   */
+  const unsigned long NO_SESSION = 0;
+  const unsigned long RECOVER_SESSION = 1;
+  const unsigned long RESUME_SESSION = 2;
+  const unsigned long DEFER_SESSION = 3;
+
+  readonly attribute unsigned long sessionType;
+  readonly attribute bool previousSessionCrashed;
+};
diff --git a/application/basilisk/components/sessionstore/nsISessionStore.idl b/application/basilisk/components/sessionstore/nsISessionStore.idl
new file mode 100644
index 0000000000..0d2500ef7c
--- /dev/null
+++ b/application/basilisk/components/sessionstore/nsISessionStore.idl
@@ -0,0 +1,220 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "nsISupports.idl"
+
+interface nsIDOMWindow;
+interface nsIDOMNode;
+
+/**
+ * nsISessionStore keeps track of the current browsing state - i.e.
+ * tab history, cookies, scroll state, form data, and window features
+ * - and allows to restore everything into one browser window.
+ *
+ * The nsISessionStore API operates mostly on browser windows and the tabbrowser
+ * tabs contained in them:
+ *
+ * * "Browser windows" are those DOM windows having loaded
+ * chrome://browser/content/browser.xul . From overlays you can just pass the
+ * global |window| object to the API, though (or |top| from a sidebar).
+ * From elsewhere you can get browser windows through the nsIWindowMediator
+ * by looking for "navigator:browser" windows.
+ *
+ * * "Tabbrowser tabs" are all the child nodes of a browser window's
+ * |gBrowser.tabContainer| such as e.g. |gBrowser.selectedTab|.
+ */
+
+[scriptable, uuid(4580f5eb-693d-423d-b0ce-2cb20a962e4d)]
+interface nsISessionStore : nsISupports
+{
+  /**
+   * Is it possible to restore the previous session. Will always be false when
+   * in Private Browsing mode.
+   */
+  attribute boolean canRestoreLastSession;
+
+  /**
+   * Restore the previous session if possible. This will not overwrite the
+   * current session. Instead the previous session will be merged into the
+   * current session. Current windows will be reused if they were windows that
+   * pinned tabs were previously restored into. New windows will be opened as
+   * needed.
+   *
+   * Note: This will throw if there is no previous state to restore. Check with
+   * canRestoreLastSession first to avoid thrown errors.
+   */
+  void restoreLastSession();
+
+  /**
+   * Get the current browsing state.
+   * @returns a JSON string representing the session state.
+   */
+  AString getBrowserState();
+
+  /**
+   * Set the browsing state.
+   * This will immediately restore the state of the whole application to the state
+   * passed in, *replacing* the current session.
+   *
+   * @param aState is a JSON string representing the session state.
+   */
+  void setBrowserState(in AString aState);
+
+  /**
+   * @param aWindow is the browser window whose state is to be returned.
+   *
+   * @returns a JSON string representing a session state with only one window.
+   */
+  AString getWindowState(in nsIDOMWindow aWindow);
+
+  /**
+   * @param aWindow    is the browser window whose state is to be set.
+   * @param aState     is a JSON string representing a session state.
+   * @param aOverwrite boolean overwrite existing tabs
+   */
+  void setWindowState(in nsIDOMWindow aWindow, in AString aState, in boolean aOverwrite);
+
+  /**
+   * @param aTab is the tabbrowser tab whose state is to be returned.
+   *
+   * @returns a JSON string representing the state of the tab
+   *         (note: doesn't contain cookies - if you need them, use getWindowState instead).
+   */
+  AString getTabState(in nsIDOMNode aTab);
+
+  /**
+   * @param aTab   is the tabbrowser tab whose state is to be set.
+   * @param aState is a JSON string representing a session state.
+   */
+  void setTabState(in nsIDOMNode aTab, in AString aState);
+
+  /**
+   * Duplicates a given tab as thoroughly as possible.
+   *
+   * @param aWindow is the browser window into which the tab will be duplicated.
+   * @param aTab    is the tabbrowser tab to duplicate (can be from a different window).
+   * @param aDelta  is the offset to the history entry to load in the duplicated tab.
+   * @returns a reference to the newly created tab.
+   */
+  nsIDOMNode duplicateTab(in nsIDOMWindow aWindow, in nsIDOMNode aTab,
+                          [optional] in long aDelta);
+
+  /**
+   * Get the number of restore-able tabs for a browser window
+   */
+  unsigned long getClosedTabCount(in nsIDOMWindow aWindow);
+
+  /**
+   * Get closed tab data
+   *
+   * @param aWindow is the browser window for which to get closed tab data
+   * @returns a JSON string representing the list of closed tabs.
+   */
+  AString getClosedTabData(in nsIDOMWindow aWindow);
+
+  /**
+   * @param aWindow is the browser window to reopen a closed tab in.
+   * @param aIndex  is the index of the tab to be restored (FIFO ordered).
+   * @returns a reference to the reopened tab.
+   */
+  nsIDOMNode undoCloseTab(in nsIDOMWindow aWindow, in unsigned long aIndex);
+
+  /**
+   * @param aWindow is the browser window associated with the closed tab.
+   * @param aIndex  is the index of the closed tab to be removed (FIFO ordered).
+   */
+  nsIDOMNode forgetClosedTab(in nsIDOMWindow aWindow, in unsigned long aIndex);
+
+  /**
+   * Get the number of restore-able windows
+   */
+  unsigned long getClosedWindowCount();
+
+  /**
+   * Get closed windows data
+   *
+   * @returns a JSON string representing the list of closed windows.
+   */
+  AString getClosedWindowData();
+
+  /**
+   * @param aIndex is the index of the windows to be restored (FIFO ordered).
+   * @returns the nsIDOMWindow object of the reopened window
+   */
+  nsIDOMWindow undoCloseWindow(in unsigned long aIndex);
+
+  /**
+   * @param aIndex  is the index of the closed window to be removed (FIFO ordered).
+   *
+   * @throws NS_ERROR_INVALID_ARG
+   *   when aIndex does not map to a closed window
+   */
+  nsIDOMNode forgetClosedWindow(in unsigned long aIndex);
+
+  /**
+   * @param aWindow is the window to get the value for.
+   * @param aKey    is the value's name.
+   *
+   * @returns A string value or an empty string if none is set.
+   */
+  AString getWindowValue(in nsIDOMWindow aWindow, in AString aKey);
+
+  /**
+   * @param aWindow      is the browser window to set the value for.
+   * @param aKey         is the value's name.
+   * @param aStringValue is the value itself (use JSON.stringify/parse before setting JS objects).
+   */
+  void setWindowValue(in nsIDOMWindow aWindow, in AString aKey, in jsval aStringValue);
+
+  /**
+   * @param aWindow is the browser window to get the value for.
+   * @param aKey    is the value's name.
+   */
+  void deleteWindowValue(in nsIDOMWindow aWindow, in AString aKey);
+
+  /**
+   * @param aTab is the tabbrowser tab to get the value for.
+   * @param aKey is the value's name.
+   *
+   * @returns A string value or an empty string if none is set.
+   */
+  AString getTabValue(in nsIDOMNode aTab, in AString aKey);
+
+  /**
+   * @param aTab         is the tabbrowser tab to set the value for.
+   * @param aKey         is the value's name.
+   * @param aStringValue is the value itself (use JSON.stringify/parse before setting JS objects).
+   */
+  void setTabValue(in nsIDOMNode aTab, in AString aKey, in jsval aStringValue);
+
+  /**
+   * @param aTab is the tabbrowser tab to get the value for.
+   * @param aKey is the value's name.
+   */
+  void deleteTabValue(in nsIDOMNode aTab, in AString aKey);
+
+   /**
+   * @param aKey is the value's name.
+   *
+   * @returns A string value or an empty string if none is set.
+   */
+  AString getGlobalValue(in AString aKey);
+
+  /**
+   * @param aKey         is the value's name.
+   * @param aStringValue is the value itself (use JSON.stringify/parse before setting JS objects).
+   */
+  void setGlobalValue(in AString aKey, in jsval aStringValue);
+
+  /**
+   * @param aTab is the browser tab to get the value for.
+   * @param aKey is the value's name.
+   */
+  void deleteGlobalValue(in AString aKey);
+
+  /**
+   * @param aName is the name of the attribute to save/restore for all tabbrowser tabs.
+   */
+  void persistTabAttribute(in AString aName);
+};
diff --git a/application/basilisk/components/sessionstore/nsSessionStartup.js b/application/basilisk/components/sessionstore/nsSessionStartup.js
new file mode 100644
index 0000000000..7593c48ece
--- /dev/null
+++ b/application/basilisk/components/sessionstore/nsSessionStartup.js
@@ -0,0 +1,353 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+"use strict";
+
+/**
+ * Session Storage and Restoration
+ *
+ * Overview
+ * This service reads user's session file at startup, and makes a determination
+ * as to whether the session should be restored. It will restore the session
+ * under the circumstances described below.  If the auto-start Private Browsing
+ * mode is active, however, the session is never restored.
+ *
+ * Crash Detection
+ * The CrashMonitor is used to check if the final session state was successfully
+ * written at shutdown of the last session. If we did not reach
+ * 'sessionstore-final-state-write-complete', then it's assumed that the browser
+ * has previously crashed and we should restore the session.
+ *
+ * Forced Restarts
+ * In the event that a restart is required due to application update or extension
+ * installation, set the browser.sessionstore.resume_session_once pref to true,
+ * and the session will be restored the next time the browser starts.
+ *
+ * Always Resume
+ * This service will always resume the session if the integer pref
+ * browser.startup.page is set to 3.
+ */
+
+/* :::::::: Constants and Helpers ::::::::::::::: */
+
+const Cc = Components.classes;
+const Ci = Components.interfaces;
+const Cr = Components.results;
+const Cu = Components.utils;
+Cu.import("resource://gre/modules/XPCOMUtils.jsm");
+Cu.import("resource://gre/modules/Services.jsm");
+Cu.import("resource://gre/modules/TelemetryStopwatch.jsm");
+Cu.import("resource://gre/modules/PrivateBrowsingUtils.jsm");
+Cu.import("resource://gre/modules/Promise.jsm");
+
+XPCOMUtils.defineLazyModuleGetter(this, "console",
+  "resource://gre/modules/Console.jsm");
+XPCOMUtils.defineLazyModuleGetter(this, "SessionFile",
+  "resource:///modules/sessionstore/SessionFile.jsm");
+XPCOMUtils.defineLazyModuleGetter(this, "StartupPerformance",
+  "resource:///modules/sessionstore/StartupPerformance.jsm");
+XPCOMUtils.defineLazyModuleGetter(this, "CrashMonitor",
+  "resource://gre/modules/CrashMonitor.jsm");
+
+const STATE_RUNNING_STR = "running";
+
+// 'browser.startup.page' preference value to resume the previous session.
+const BROWSER_STARTUP_RESUME_SESSION = 3;
+
+function debug(aMsg) {
+  aMsg = ("SessionStartup: " + aMsg).replace(/\S{80}/g, "$&\n");
+  Services.console.logStringMessage(aMsg);
+}
+function warning(aMsg, aException) {
+  let consoleMsg = Cc["@mozilla.org/scripterror;1"].createInstance(Ci.nsIScriptError);
+consoleMsg.init(aMsg, aException.fileName, null, aException.lineNumber, 0, Ci.nsIScriptError.warningFlag, "component javascript");
+  Services.console.logMessage(consoleMsg);
+}
+
+var gOnceInitializedDeferred = (function () {
+  let deferred = {};
+
+  deferred.promise = new Promise((resolve, reject) => {
+    deferred.resolve = resolve;
+    deferred.reject = reject;
+  });
+
+  return deferred;
+})();
+
+/* :::::::: The Service ::::::::::::::: */
+
+function SessionStartup() {
+}
+
+SessionStartup.prototype = {
+
+  // the state to restore at startup
+  _initialState: null,
+  _sessionType: Ci.nsISessionStartup.NO_SESSION,
+  _initialized: false,
+
+  // Stores whether the previous session crashed.
+  _previousSessionCrashed: null,
+
+/* ........ Global Event Handlers .............. */
+
+  /**
+   * Initialize the component
+   */
+  init: function sss_init() {
+    Services.obs.notifyObservers(null, "sessionstore-init-started", null);
+    StartupPerformance.init();
+
+    // do not need to initialize anything in auto-started private browsing sessions
+    if (PrivateBrowsingUtils.permanentPrivateBrowsing) {
+      this._initialized = true;
+      gOnceInitializedDeferred.resolve();
+      return;
+    }
+
+    SessionFile.read().then(
+      this._onSessionFileRead.bind(this),
+      console.error
+    );
+  },
+
+  // Wrap a string as a nsISupports
+  _createSupportsString: function ssfi_createSupportsString(aData) {
+    let string = Cc["@mozilla.org/supports-string;1"]
+                   .createInstance(Ci.nsISupportsString);
+    string.data = aData;
+    return string;
+  },
+
+  /**
+   * Complete initialization once the Session File has been read
+   *
+   * @param source The Session State string read from disk.
+   * @param parsed The object obtained by parsing |source| as JSON.
+   */
+  _onSessionFileRead: function ({source, parsed, noFilesFound}) {
+    this._initialized = true;
+
+    // Let observers modify the state before it is used
+    let supportsStateString = this._createSupportsString(source);
+    Services.obs.notifyObservers(supportsStateString, "sessionstore-state-read", "");
+    let stateString = supportsStateString.data;
+
+    if (stateString != source) {
+      // The session has been modified by an add-on, reparse.
+      try {
+        this._initialState = JSON.parse(stateString);
+      } catch (ex) {
+        // That's not very good, an add-on has rewritten the initial
+        // state to something that won't parse.
+        warning("Observer rewrote the state to something that won't parse", ex);
+      }
+    } else {
+      // No need to reparse
+      this._initialState = parsed;
+    }
+
+    if (this._initialState == null) {
+      // No valid session found.
+      this._sessionType = Ci.nsISessionStartup.NO_SESSION;
+      Services.obs.notifyObservers(null, "sessionstore-state-finalized", "");
+      gOnceInitializedDeferred.resolve();
+      return;
+    }
+
+    let shouldResumeSessionOnce = Services.prefs.getBoolPref("browser.sessionstore.resume_session_once");
+    let shouldResumeSession = shouldResumeSessionOnce ||
+          Services.prefs.getIntPref("browser.startup.page") == BROWSER_STARTUP_RESUME_SESSION;
+
+    // If this is a normal restore then throw away any previous session
+    if (!shouldResumeSessionOnce && this._initialState) {
+      delete this._initialState.lastSessionState;
+    }
+
+    let resumeFromCrash = Services.prefs.getBoolPref("browser.sessionstore.resume_from_crash");
+
+    CrashMonitor.previousCheckpoints.then(checkpoints => {
+      if (checkpoints) {
+        // If the previous session finished writing the final state, we'll
+        // assume there was no crash.
+        this._previousSessionCrashed = !checkpoints["sessionstore-final-state-write-complete"];
+
+      } else {
+        // If the Crash Monitor could not load a checkpoints file it will
+        // provide null. This could occur on the first run after updating to
+        // a version including the Crash Monitor, or if the checkpoints file
+        // was removed, or on first startup with this profile, or after Firefox Reset.
+
+        if (noFilesFound) {
+          // There was no checkpoints file and no sessionstore.js or its backups
+          // so we will assume that this was a fresh profile.
+          this._previousSessionCrashed = false;
+
+        } else {
+          // If this is the first run after an update, sessionstore.js should
+          // still contain the session.state flag to indicate if the session
+          // crashed. If it is not present, we will assume this was not the first
+          // run after update and the checkpoints file was somehow corrupted or
+          // removed by a crash.
+          //
+          // If the session.state flag is present, we will fallback to using it
+          // for crash detection - If the last write of sessionstore.js had it
+          // set to "running", we crashed.
+          let stateFlagPresent = (this._initialState.session &&
+                                  this._initialState.session.state);
+
+
+          this._previousSessionCrashed = !stateFlagPresent ||
+            (this._initialState.session.state == STATE_RUNNING_STR);
+        }
+      }
+
+      // Report shutdown success via telemetry. Shortcoming here are
+      // being-killed-by-OS-shutdown-logic, shutdown freezing after
+      // session restore was written, etc.
+      Services.telemetry.getHistogramById("SHUTDOWN_OK").add(!this._previousSessionCrashed);
+
+      // set the startup type
+      if (this._previousSessionCrashed && resumeFromCrash)
+        this._sessionType = Ci.nsISessionStartup.RECOVER_SESSION;
+      else if (!this._previousSessionCrashed && shouldResumeSession)
+        this._sessionType = Ci.nsISessionStartup.RESUME_SESSION;
+      else if (this._initialState)
+        this._sessionType = Ci.nsISessionStartup.DEFER_SESSION;
+      else
+        this._initialState = null; // reset the state
+
+      Services.obs.addObserver(this, "sessionstore-windows-restored", true);
+
+      if (this._sessionType != Ci.nsISessionStartup.NO_SESSION)
+        Services.obs.addObserver(this, "browser:purge-session-history", true);
+
+      // We're ready. Notify everyone else.
+      Services.obs.notifyObservers(null, "sessionstore-state-finalized", "");
+      gOnceInitializedDeferred.resolve();
+    });
+  },
+
+  /**
+   * Handle notifications
+   */
+  observe: function sss_observe(aSubject, aTopic, aData) {
+    switch (aTopic) {
+    case "app-startup":
+      Services.obs.addObserver(this, "final-ui-startup", true);
+      Services.obs.addObserver(this, "quit-application", true);
+      break;
+    case "final-ui-startup":
+      Services.obs.removeObserver(this, "final-ui-startup");
+      Services.obs.removeObserver(this, "quit-application");
+      this.init();
+      break;
+    case "quit-application":
+      // no reason for initializing at this point (cf. bug 409115)
+      Services.obs.removeObserver(this, "final-ui-startup");
+      Services.obs.removeObserver(this, "quit-application");
+      if (this._sessionType != Ci.nsISessionStartup.NO_SESSION)
+        Services.obs.removeObserver(this, "browser:purge-session-history");
+      break;
+    case "sessionstore-windows-restored":
+      Services.obs.removeObserver(this, "sessionstore-windows-restored");
+      // free _initialState after nsSessionStore is done with it
+      this._initialState = null;
+      break;
+    case "browser:purge-session-history":
+      Services.obs.removeObserver(this, "browser:purge-session-history");
+      // reset all state on sanitization
+      this._sessionType = Ci.nsISessionStartup.NO_SESSION;
+      break;
+    }
+  },
+
+/* ........ Public API ................*/
+
+  get onceInitialized() {
+    return gOnceInitializedDeferred.promise;
+  },
+
+  /**
+   * Get the session state as a jsval
+   */
+  get state() {
+    return this._initialState;
+  },
+
+  /**
+   * Determines whether there is a pending session restore. Should only be
+   * called after initialization has completed.
+   * @returns bool
+   */
+  doRestore: function sss_doRestore() {
+    return this._willRestore();
+  },
+
+  /**
+   * Determines whether automatic session restoration is enabled for this
+   * launch of the browser. This does not include crash restoration. In
+   * particular, if session restore is configured to restore only in case of
+   * crash, this method returns false.
+   * @returns bool
+   */
+  isAutomaticRestoreEnabled: function () {
+    return Services.prefs.getBoolPref("browser.sessionstore.resume_session_once") ||
+           Services.prefs.getIntPref("browser.startup.page") == BROWSER_STARTUP_RESUME_SESSION;
+  },
+
+  /**
+   * Determines whether there is a pending session restore.
+   * @returns bool
+   */
+  _willRestore: function () {
+    return this._sessionType == Ci.nsISessionStartup.RECOVER_SESSION ||
+           this._sessionType == Ci.nsISessionStartup.RESUME_SESSION;
+  },
+
+  /**
+   * Returns whether we will restore a session that ends up replacing the
+   * homepage. The browser uses this to not start loading the homepage if
+   * we're going to stop its load anyway shortly after.
+   *
+   * This is meant to be an optimization for the average case that loading the
+   * session file finishes before we may want to start loading the default
+   * homepage. Should this be called before the session file has been read it
+   * will just return false.
+   *
+   * @returns bool
+   */
+  get willOverrideHomepage() {
+    if (this._initialState && this._willRestore()) {
+      let windows = this._initialState.windows || null;
+      // If there are valid windows with not only pinned tabs, signal that we
+      // will override the default homepage by restoring a session.
+      return windows && windows.some(w => w.tabs.some(t => !t.pinned));
+    }
+    return false;
+  },
+
+  /**
+   * Get the type of pending session store, if any.
+   */
+  get sessionType() {
+    return this._sessionType;
+  },
+
+  /**
+   * Get whether the previous session crashed.
+   */
+  get previousSessionCrashed() {
+    return this._previousSessionCrashed;
+  },
+
+  /* ........ QueryInterface .............. */
+  QueryInterface : XPCOMUtils.generateQI([Ci.nsIObserver,
+                                          Ci.nsISupportsWeakReference,
+                                          Ci.nsISessionStartup]),
+  classID:          Components.ID("{ec7a6c20-e081-11da-8ad9-0800200c9a66}")
+};
+
+this.NSGetFactory = XPCOMUtils.generateNSGetFactory([SessionStartup]);
diff --git a/application/basilisk/components/sessionstore/nsSessionStore.js b/application/basilisk/components/sessionstore/nsSessionStore.js
new file mode 100644
index 0000000000..8d96178ce3
--- /dev/null
+++ b/application/basilisk/components/sessionstore/nsSessionStore.js
@@ -0,0 +1,39 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+"use strict";
+
+/**
+ * Session Storage and Restoration
+ *
+ * Overview
+ * This service keeps track of a user's session, storing the various bits
+ * required to return the browser to its current state. The relevant data is
+ * stored in memory, and is periodically saved to disk in a file in the
+ * profile directory. The service is started at first window load, in
+ * delayedStartup, and will restore the session from the data received from
+ * the nsSessionStartup service.
+ */
+
+const Cu = Components.utils;
+const Ci = Components.interfaces;
+
+Cu.import("resource://gre/modules/XPCOMUtils.jsm");
+Cu.import("resource:///modules/sessionstore/SessionStore.jsm");
+
+function SessionStoreService() {}
+
+// The SessionStore module's object is frozen. We need to modify our prototype
+// and add some properties so let's just copy the SessionStore object.
+Object.keys(SessionStore).forEach(function (aName) {
+  let desc = Object.getOwnPropertyDescriptor(SessionStore, aName);
+  Object.defineProperty(SessionStoreService.prototype, aName, desc);
+});
+
+SessionStoreService.prototype.classID =
+  Components.ID("{5280606b-2510-4fe0-97ef-9b5a22eafe6b}");
+SessionStoreService.prototype.QueryInterface =
+  XPCOMUtils.generateQI([Ci.nsISessionStore]);
+
+this.NSGetFactory = XPCOMUtils.generateNSGetFactory([SessionStoreService]);
diff --git a/application/basilisk/components/sessionstore/nsSessionStore.manifest b/application/basilisk/components/sessionstore/nsSessionStore.manifest
new file mode 100644
index 0000000000..9b5819c6a3
--- /dev/null
+++ b/application/basilisk/components/sessionstore/nsSessionStore.manifest
@@ -0,0 +1,15 @@
+# This component must restrict its registration for the app-startup category
+# to the specific list of apps that use it so it doesn't get loaded in xpcshell.
+# Thus we restrict it to these apps:
+#
+#   b2g:            {3c2e2abc-06d4-11e1-ac3b-374f68613e61}
+#   browser:        {ec8030f7-c20a-464f-9b0e-13a3a9e97384}
+#   mobile/android: {aa3c5121-dab2-40e2-81ca-7ea25febc110}
+#   mobile/xul:     {a23983c0-fd0e-11dc-95ff-0800200c9a66}
+#   graphene:       {d1bfe7d9-c01e-4237-998b-7b5f960a4314}
+
+component {5280606b-2510-4fe0-97ef-9b5a22eafe6b} nsSessionStore.js
+contract @mozilla.org/browser/sessionstore;1 {5280606b-2510-4fe0-97ef-9b5a22eafe6b}
+component {ec7a6c20-e081-11da-8ad9-0800200c9a66} nsSessionStartup.js
+contract @mozilla.org/browser/sessionstartup;1 {ec7a6c20-e081-11da-8ad9-0800200c9a66}
+category app-startup nsSessionStartup service,@mozilla.org/browser/sessionstartup;1 application={3c2e2abc-06d4-11e1-ac3b-374f68613e61} application={ec8030f7-c20a-464f-9b0e-13a3a9e97384} application={aa3c5121-dab2-40e2-81ca-7ea25febc110} application={a23983c0-fd0e-11dc-95ff-0800200c9a66} application={d1bfe7d9-c01e-4237-998b-7b5f960a4314}