Issue #2343 - replace MOZ_FALLTHROUGH with [[fallthrough]]

Basically a S&R. Removed the macro and adjusts IDL codegen accordingly.

138 files changed, 299 insertions, 343 deletions

diff --git a/dom/animation/TimingParams.cpp b/dom/animation/TimingParams.cpp
index f387f460f8..8c64224a6e 100644
--- a/dom/animation/TimingParams.cpp
+++ b/dom/animation/TimingParams.cpp
@@ -135,7 +135,7 @@ TimingParams::ParseEasing(const nsAString& aEasing,
               NS_STYLE_TRANSITION_TIMING_FUNCTION_LINEAR) {
             return Nothing();
           }
-          MOZ_FALLTHROUGH;
+          [[fallthrough]];
         case eCSSUnit_Cubic_Bezier:
         case eCSSUnit_Steps: {
           nsTimingFunction timingFunction;
diff --git a/dom/base/Element.cpp b/dom/base/Element.cpp
index c67e42bd70..89d37b5787 100644
--- a/dom/base/Element.cpp
+++ b/dom/base/Element.cpp
@@ -3199,7 +3199,7 @@ Element::GetEventTargetParentForLinks(EventChainPreVisitor& aVisitor)
   // Set the status bar similarly for mouseover and focus
   case eMouseOver:
     aVisitor.mEventStatus = nsEventStatus_eConsumeNoDefault;
-    MOZ_FALLTHROUGH;
+    [[fallthrough]];
   case eFocus: {
     InternalFocusEvent* focusEvent = aVisitor.mEvent->AsFocusEvent();
     if (!focusEvent || !focusEvent->mIsRefocus) {
@@ -3214,7 +3214,7 @@ Element::GetEventTargetParentForLinks(EventChainPreVisitor& aVisitor)
   }
   case eMouseOut:
     aVisitor.mEventStatus = nsEventStatus_eConsumeNoDefault;
-    MOZ_FALLTHROUGH;
+    [[fallthrough]];
   case eBlur:
     rv = LeaveLink(aVisitor.mPresContext);
     if (NS_SUCCEEDED(rv)) {
diff --git a/dom/base/nsDocument.cpp b/dom/base/nsDocument.cpp
index 9b7a8c2721..4d34303028 100644
--- a/dom/base/nsDocument.cpp
+++ b/dom/base/nsDocument.cpp
@@ -5787,7 +5787,7 @@ nsIDocument::ImportNode(nsINode& aNode, bool aDeep, ErrorResult& rv) const
       if (ShadowRoot::FromNode(imported)) {
         break;
       }
-      MOZ_FALLTHROUGH;
+      [[fallthrough]];
     }
     case nsIDOMNode::ATTRIBUTE_NODE:
     case nsIDOMNode::ELEMENT_NODE:
@@ -6893,7 +6893,7 @@ nsIDocument::AdoptNode(nsINode& aAdoptedNode, ErrorResult& rv)
         rv.Throw(NS_ERROR_DOM_HIERARCHY_REQUEST_ERR);
         return nullptr;
       }
-      MOZ_FALLTHROUGH;
+      [[fallthrough]];
     }
     case nsIDOMNode::ELEMENT_NODE:
     case nsIDOMNode::PROCESSING_INSTRUCTION_NODE:
@@ -7184,7 +7184,7 @@ nsDocument::GetViewportInfo(const ScreenIntSize& aDisplaySize)
     mValidMaxScale = !maxScaleStr.IsEmpty() && NS_SUCCEEDED(scaleMaxErrorCode);
 
     mViewportType = Specified;
-    MOZ_FALLTHROUGH;
+    [[fallthrough]];
   }
   case Specified:
   default:
diff --git a/dom/base/nsXMLContentSerializer.cpp b/dom/base/nsXMLContentSerializer.cpp
index 1bf075875f..e5cf12f638 100644
--- a/dom/base/nsXMLContentSerializer.cpp
+++ b/dom/base/nsXMLContentSerializer.cpp
@@ -1442,7 +1442,7 @@ nsXMLContentSerializer::AppendFormatedWrapped_WhitespaceSequence(
       case ' ':
       case '\t':
         sawBlankOrTab = true;
-        MOZ_FALLTHROUGH;
+        [[fallthrough]];
       case '\n':
         ++aPos;
         // do not increase mColPos,
diff --git a/dom/bindings/Codegen.py b/dom/bindings/Codegen.py
index b7caaad7bb..d73d9fde63 100644
--- a/dom/bindings/Codegen.py
+++ b/dom/bindings/Codegen.py
@@ -7918,7 +7918,7 @@ class CGCase(CGList):
         self.append(CGGeneric("case " + expression + ": {\n"))
         bodyList = CGList([body])
         if fallThrough:
-            bodyList.append(CGGeneric("MOZ_FALLTHROUGH;\n"))
+            bodyList.append(CGGeneric("[[fallthrough]];\n"))
         else:
             bodyList.append(CGGeneric("break;\n"))
         self.append(CGIndenter(bodyList))
diff --git a/dom/canvas/WebGL2ContextState.cpp b/dom/canvas/WebGL2ContextState.cpp
index 5cdfb7277b..57f818c524 100644
--- a/dom/canvas/WebGL2ContextState.cpp
+++ b/dom/canvas/WebGL2ContextState.cpp
@@ -121,7 +121,7 @@ WebGL2Context::GetParameter(JSContext* cx, GLenum pname, ErrorResult& rv)
         return JS::NumberValue(UINT32_MAX);
 
       /*** fall through to fGetInteger64v ***/
-      MOZ_FALLTHROUGH;
+      [[fallthrough]];
 
     case LOCAL_GL_MAX_COMBINED_FRAGMENT_UNIFORM_COMPONENTS:
     case LOCAL_GL_MAX_COMBINED_VERTEX_UNIFORM_COMPONENTS:
diff --git a/dom/canvas/WebGLContextGL.cpp b/dom/canvas/WebGLContextGL.cpp
index d01a0d44e3..9e7ac963ca 100644
--- a/dom/canvas/WebGLContextGL.cpp
+++ b/dom/canvas/WebGLContextGL.cpp
@@ -792,7 +792,7 @@ WebGLContext::GetRenderbufferParameter(GLenum target, GLenum pname)
     case LOCAL_GL_RENDERBUFFER_SAMPLES:
         if (!IsWebGL2())
             break;
-        MOZ_FALLTHROUGH;
+        [[fallthrough]];
 
     case LOCAL_GL_RENDERBUFFER_WIDTH:
     case LOCAL_GL_RENDERBUFFER_HEIGHT:
diff --git a/dom/canvas/WebGLQuery.cpp b/dom/canvas/WebGLQuery.cpp
index ea71243c97..60a5d0bc76 100644
--- a/dom/canvas/WebGLQuery.cpp
+++ b/dom/canvas/WebGLQuery.cpp
@@ -178,7 +178,7 @@ WebGLQuery::GetQueryParameter(GLenum pname, JS::MutableHandleValue retval) const
                 gl->fGetQueryObjectui64v(mGLName, pname, &val);
                 break;
             }
-            MOZ_FALLTHROUGH;
+            [[fallthrough]];
 
         default:
             gl->fGetQueryObjectuiv(mGLName, LOCAL_GL_QUERY_RESULT, (GLuint*)&val);
diff --git a/dom/events/EventStateManager.cpp b/dom/events/EventStateManager.cpp
index 935281916b..c1418e5375 100644
--- a/dom/events/EventStateManager.cpp
+++ b/dom/events/EventStateManager.cpp
@@ -654,7 +654,7 @@ EventStateManager::PreHandleEvent(nsPresContext* aPresContext,
         StopTrackingDragGesture();
         sNormalLMouseEventInProcess = false;
         // then fall through...
-        MOZ_FALLTHROUGH;
+        [[fallthrough]];
       case WidgetMouseEvent::eRightButton:
       case WidgetMouseEvent::eMiddleButton:
         SetClickCount(mouseEvent, aStatus);
@@ -708,7 +708,7 @@ EventStateManager::PreHandleEvent(nsPresContext* aPresContext,
       aEvent->mMessage = eVoidEvent;
       break;
     }
-    MOZ_FALLTHROUGH;
+    [[fallthrough]];
   case eMouseMove:
   case ePointerDown:
   case ePointerMove: {
@@ -774,7 +774,7 @@ EventStateManager::PreHandleEvent(nsPresContext* aPresContext,
       }
     }
     // then fall through...
-    MOZ_FALLTHROUGH;
+    [[fallthrough]];
   case eBeforeKeyDown:
   case eKeyDown:
   case eAfterKeyDown:
@@ -3153,7 +3153,7 @@ EventStateManager::PostHandleEvent(nsPresContext* aPresContext,
     }
     // After firing the pointercancel event, a user agent must also fire a
     // pointerout event followed by a pointerleave event.
-    MOZ_FALLTHROUGH;
+    [[fallthrough]];
   }
   case ePointerUp: {
     WidgetPointerEvent* pointerEvent = aEvent->AsPointerEvent();
@@ -4272,7 +4272,7 @@ EventStateManager::GenerateMouseEnterExit(WidgetMouseEvent* aMouseEvent)
       // Update the last known mRefPoint with the current mRefPoint.
       sLastRefPoint = aMouseEvent->mRefPoint;
     }
-    MOZ_FALLTHROUGH;
+    [[fallthrough]];
   case ePointerMove:
   case ePointerDown:
   case ePointerGotCapture:
diff --git a/dom/html/HTMLInputElement.cpp b/dom/html/HTMLInputElement.cpp
index 3d0f643712..c01c3f7017 100644
--- a/dom/html/HTMLInputElement.cpp
+++ b/dom/html/HTMLInputElement.cpp
@@ -4545,7 +4545,7 @@ HTMLInputElement::PostHandleEvent(EventChainPostVisitor& aVisitor)
                   break;  // If we are submitting, do not send click event
                 }
                 // else fall through and treat Space like click...
-                MOZ_FALLTHROUGH;
+                [[fallthrough]];
               }
               case NS_FORM_INPUT_BUTTON:
               case NS_FORM_INPUT_RESET:
@@ -4567,7 +4567,7 @@ HTMLInputElement::PostHandleEvent(EventChainPostVisitor& aVisitor)
               case NS_VK_UP:
               case NS_VK_LEFT:
                 isMovingBack = true;
-                MOZ_FALLTHROUGH;
+                [[fallthrough]];
               case NS_VK_DOWN:
               case NS_VK_RIGHT:
               // Arrow key pressed, focus+select prev/next radio button
diff --git a/dom/media/MediaInfo.cpp b/dom/media/MediaInfo.cpp
index 568183e688..f40f1584d9 100644
--- a/dom/media/MediaInfo.cpp
+++ b/dom/media/MediaInfo.cpp
@@ -144,9 +144,9 @@ AudioConfig::SampleSize(AudioConfig::SampleFormat aFormat)
   switch (aFormat) {
     case FORMAT_U8:     return 1;
     case FORMAT_S16:    return 2;
-    case FORMAT_S24:    MOZ_FALLTHROUGH;
-    case FORMAT_S24LSB: MOZ_FALLTHROUGH;
-    case FORMAT_S32:    MOZ_FALLTHROUGH;
+    case FORMAT_S24:    [[fallthrough]];
+    case FORMAT_S24LSB: [[fallthrough]];
+    case FORMAT_S32:    [[fallthrough]];
     case FORMAT_FLT:    return 4;
     case FORMAT_NONE:
     default:            return 0;
@@ -159,11 +159,11 @@ AudioConfig::FormatToBits(AudioConfig::SampleFormat aFormat)
   switch (aFormat) {
     case FORMAT_U8:     return 8;
     case FORMAT_S16:    return 16;
-    case FORMAT_S24LSB: MOZ_FALLTHROUGH;
+    case FORMAT_S24LSB: [[fallthrough]];
     case FORMAT_S24:    return 24;
-    case FORMAT_S32:    MOZ_FALLTHROUGH;
+    case FORMAT_S32:    [[fallthrough]];
     case FORMAT_FLT:    return 32;
-    case FORMAT_NONE:   MOZ_FALLTHROUGH;
+    case FORMAT_NONE:   [[fallthrough]];
     default:            return 0;
   }
 }
diff --git a/dom/media/MediaManager.cpp b/dom/media/MediaManager.cpp
index 7fd11f1715..b98e0cfb06 100644
--- a/dom/media/MediaManager.cpp
+++ b/dom/media/MediaManager.cpp
@@ -2053,7 +2053,7 @@ MediaManager::GetUserMedia(nsPIDOMWindowInner* aWindow,
           nsPIDOMWindowOuter* outer = aWindow->GetOuterWindow();
           vc.mBrowserWindow.Construct(outer->WindowID());
         }
-        MOZ_FALLTHROUGH;
+        [[fallthrough]];
       case MediaSourceEnum::Screen:
       case MediaSourceEnum::Application:
       case MediaSourceEnum::Window:
diff --git a/dom/media/fmp4/MP4Demuxer.cpp b/dom/media/fmp4/MP4Demuxer.cpp
index 5ac7fa518c..77eb783a28 100644
--- a/dom/media/fmp4/MP4Demuxer.cpp
+++ b/dom/media/fmp4/MP4Demuxer.cpp
@@ -263,7 +263,7 @@ MP4TrackDemuxer::GetNextSample()
       mp4_demuxer::H264::FrameType type =
         mp4_demuxer::H264::GetFrameType(sample);
       switch (type) {
-        case mp4_demuxer::H264::FrameType::I_FRAME: MOZ_FALLTHROUGH;
+        case mp4_demuxer::H264::FrameType::I_FRAME: [[fallthrough]];
         case mp4_demuxer::H264::FrameType::OTHER:
         {
           bool keyframe = type == mp4_demuxer::H264::FrameType::I_FRAME;
diff --git a/dom/media/webm/WebMBufferedParser.cpp b/dom/media/webm/WebMBufferedParser.cpp
index 8d9bd7d811..7fc5bc2913 100644
--- a/dom/media/webm/WebMBufferedParser.cpp
+++ b/dom/media/webm/WebMBufferedParser.cpp
@@ -141,7 +141,7 @@ bool WebMBufferedParser::Append(const unsigned char* aBuffer, uint32_t aLength,
       case EBML_ID:
         mLastInitStartOffset = mCurrentOffset + (p - aBuffer) -
                             (mElement.mID.mLength + mElement.mSize.mLength);
-        MOZ_FALLTHROUGH;
+        [[fallthrough]];
       default:
         mSkipBytes = mElement.mSize.mValue;
         mState = SKIP_DATA;
diff --git a/dom/plugins/base/nsNPAPIPlugin.cpp b/dom/plugins/base/nsNPAPIPlugin.cpp
index 4b02440e20..aac2261196 100644
--- a/dom/plugins/base/nsNPAPIPlugin.cpp
+++ b/dom/plugins/base/nsNPAPIPlugin.cpp
@@ -2022,7 +2022,7 @@ _getvalue(NPP npp, NPNVariable variable, void *result)
     // old XPCOM objects, no longer supported, but null out the out
     // param to avoid crashing plugins that still try to use this.
     *(nsISupports**)result = nullptr;
-    MOZ_FALLTHROUGH;
+    [[fallthrough]];
 
   default:
     NPN_PLUGIN_LOG(PLUGIN_LOG_NORMAL, ("NPN_getvalue unhandled get value: %d\n", variable));
diff --git a/dom/plugins/ipc/PluginInstanceChild.cpp b/dom/plugins/ipc/PluginInstanceChild.cpp
index 094d96d380..5a48c8cd47 100644
--- a/dom/plugins/ipc/PluginInstanceChild.cpp
+++ b/dom/plugins/ipc/PluginInstanceChild.cpp
@@ -561,7 +561,7 @@ PluginInstanceChild::NPN_GetValue(NPNVariable aVar,
     case NPNVSupportsXEmbedBool:
     case NPNVSupportsWindowless:
         NS_NOTREACHED("NPNVariable should be handled in PluginModuleChild.");
-        MOZ_FALLTHROUGH;
+        [[fallthrough]];
 #endif
 
     default:
diff --git a/dom/smil/nsSMILParserUtils.cpp b/dom/smil/nsSMILParserUtils.cpp
index 05b8fd7ba8..477c538c7e 100644
--- a/dom/smil/nsSMILParserUtils.cpp
+++ b/dom/smil/nsSMILParserUtils.cpp
@@ -186,7 +186,7 @@ ParseClockValue(RangedPtr<const char16_t>& aIter,
           !ParseColon(iter, aEnd)) {
         return false;
       }
-      MOZ_FALLTHROUGH;
+      [[fallthrough]];
     case PARTIAL_CLOCK_VALUE:
       if (!ParseSecondsOrMinutes(iter, aEnd, minutes) ||
           !ParseColon(iter, aEnd) ||
diff --git a/dom/storage/DOMStorageDBThread.cpp b/dom/storage/DOMStorageDBThread.cpp
index 9c19c9f731..fb20f35350 100644
--- a/dom/storage/DOMStorageDBThread.cpp
+++ b/dom/storage/DOMStorageDBThread.cpp
@@ -284,7 +284,7 @@ DOMStorageDBThread::InsertDBOp(DOMStorageDBThread::DBOperation* aOperation)
       aOperation->Finalize(NS_OK);
       return NS_OK;
     }
-    MOZ_FALLTHROUGH;
+    [[fallthrough]];
 
   case DBOperation::opGetUsage:
     if (aOperation->Type() == DBOperation::opPreloadUrgent) {
diff --git a/dom/storage/DOMStorageDBUpdater.cpp b/dom/storage/DOMStorageDBUpdater.cpp
index f7aa0310f8..696d5da7e8 100644
--- a/dom/storage/DOMStorageDBUpdater.cpp
+++ b/dom/storage/DOMStorageDBUpdater.cpp
@@ -385,7 +385,7 @@ nsresult Update(mozIStorageConnection *aWorkerConnection)
     rv = aWorkerConnection->SetSchemaVersion(1);
     NS_ENSURE_SUCCESS(rv, rv);
 
-    MOZ_FALLTHROUGH;
+    [[fallthrough]];
   }
   case CURRENT_SCHEMA_VERSION:
     // Ensure the tables and indexes are up.  This is mostly a no-op
diff --git a/dom/svg/SVGPathSegListSMILType.cpp b/dom/svg/SVGPathSegListSMILType.cpp
index ba42f494e9..1f03fb8fd6 100644
--- a/dom/svg/SVGPathSegListSMILType.cpp
+++ b/dom/svg/SVGPathSegListSMILType.cpp
@@ -351,7 +351,7 @@ ConvertPathSegmentData(SVGPathDataAndInfo::const_iterator& aStart,
       aResult[5] = aStart[5];
       aResult[6] = aStart[6];
       AdjustSegmentForRelativeness(adjustmentType, aResult + 5, aState);
-      MOZ_FALLTHROUGH;
+      [[fallthrough]];
     case PATHSEG_CURVETO_QUADRATIC_ABS:
     case PATHSEG_CURVETO_QUADRATIC_REL:
     case PATHSEG_CURVETO_CUBIC_SMOOTH_ABS:
@@ -359,7 +359,7 @@ ConvertPathSegmentData(SVGPathDataAndInfo::const_iterator& aStart,
       aResult[3] = aStart[3];
       aResult[4] = aStart[4];
       AdjustSegmentForRelativeness(adjustmentType, aResult + 3, aState);
-      MOZ_FALLTHROUGH;
+      [[fallthrough]];
     case PATHSEG_MOVETO_ABS:
     case PATHSEG_MOVETO_REL:
     case PATHSEG_LINETO_ABS:
diff --git a/dom/svg/SVGTransformListParser.cpp b/dom/svg/SVGTransformListParser.cpp
index 77f9fcbbbc..67eb6e513b 100644
--- a/dom/svg/SVGTransformListParser.cpp
+++ b/dom/svg/SVGTransformListParser.cpp
@@ -142,7 +142,7 @@ SVGTransformListParser::ParseTranslate()
   switch (count) {
     case 1:
       t[1] = 0.f;
-      MOZ_FALLTHROUGH;
+      [[fallthrough]];
     case 2:
     {
       nsSVGTransform* transform = mTransforms.AppendElement(fallible);
@@ -170,7 +170,7 @@ SVGTransformListParser::ParseScale()
   switch (count) {
     case 1:
       s[1] = s[0];
-      MOZ_FALLTHROUGH;
+      [[fallthrough]];
     case 2:
     {
       nsSVGTransform* transform = mTransforms.AppendElement(fallible);
@@ -199,7 +199,7 @@ SVGTransformListParser::ParseRotate()
   switch (count) {
     case 1:
       r[1] = r[2] = 0.f;
-      MOZ_FALLTHROUGH;
+      [[fallthrough]];
     case 3:
     {
       nsSVGTransform* transform = mTransforms.AppendElement(fallible);
diff --git a/dom/system/linux/GpsdLocationProvider.cpp b/dom/system/linux/GpsdLocationProvider.cpp
index c117188692..38d5089a50 100644
--- a/dom/system/linux/GpsdLocationProvider.cpp
+++ b/dom/system/linux/GpsdLocationProvider.cpp
@@ -255,7 +255,7 @@ protected:
           if (!IsNaN(gpsData.fix.altitude)) {
             alt = gpsData.fix.altitude;
           }
-          MOZ_FALLTHROUGH;
+          [[fallthrough]];
         case MODE_2D:
           if (!IsNaN(gpsData.fix.latitude)) {
             lat = gpsData.fix.latitude;
@@ -308,13 +308,13 @@ protected:
   {
     switch (aErrno) {
       case EACCES:
-          MOZ_FALLTHROUGH;
+          [[fallthrough]];
       case EPERM:
-          MOZ_FALLTHROUGH;
+          [[fallthrough]];
       case EROFS:
         return nsIDOMGeoPositionError::PERMISSION_DENIED;
       case ETIME:
-          MOZ_FALLTHROUGH;
+          [[fallthrough]];
       case ETIMEDOUT:
         return nsIDOMGeoPositionError::TIMEOUT;
       default:
diff --git a/dom/xslt/xpath/txLocationStep.cpp b/dom/xslt/xpath/txLocationStep.cpp
index f945b750fe..0625d3944f 100644
--- a/dom/xslt/xpath/txLocationStep.cpp
+++ b/dom/xslt/xpath/txLocationStep.cpp
@@ -42,7 +42,7 @@ LocationStep::evaluate(txIEvalContext* aContext, txAExprResult** aResult)
             if (!walker.moveToParent()) {
                 break;
             }
-            MOZ_FALLTHROUGH;
+            [[fallthrough]];
         }
         case ANCESTOR_OR_SELF_AXIS:
         {
@@ -74,7 +74,7 @@ LocationStep::evaluate(txIEvalContext* aContext, txAExprResult** aResult)
             if (mNodeTest->matches(walker.getCurrentPosition(), aContext)) {
                 nodes->append(walker.getCurrentPosition());
             }
-            MOZ_FALLTHROUGH;
+            [[fallthrough]];
         }
         case DESCENDANT_AXIS:
         {
diff --git a/dom/xslt/xslt/txOutputFormat.cpp b/dom/xslt/xslt/txOutputFormat.cpp
index dcf28f5cb5..3609c5c994 100644
--- a/dom/xslt/xslt/txOutputFormat.cpp
+++ b/dom/xslt/xslt/txOutputFormat.cpp
@@ -81,7 +81,7 @@ void txOutputFormat::setFromDefaults()
         case eMethodNotSet:
         {
             mMethod = eXMLOutput;
-            MOZ_FALLTHROUGH;
+            [[fallthrough]];
         }
         case eXMLOutput:
         {
diff --git a/extensions/cookie/nsCookiePermission.cpp b/extensions/cookie/nsCookiePermission.cpp
index 21e5553766..840523a31a 100644
--- a/extensions/cookie/nsCookiePermission.cpp
+++ b/extensions/cookie/nsCookiePermission.cpp
@@ -190,7 +190,7 @@ nsCookiePermission::CanSetCookie(nsIURI     *aURI,
   switch (perm) {
   case nsICookiePermission::ACCESS_SESSION:
     *aIsSession = true;
-    MOZ_FALLTHROUGH;
+    [[fallthrough]];
 
   case nsICookiePermission::ACCESS_ALLOW:
     *aResult = true;
diff --git a/extensions/cookie/nsPermissionManager.cpp b/extensions/cookie/nsPermissionManager.cpp
index afdaea63f1..9a3426f635 100644
--- a/extensions/cookie/nsPermissionManager.cpp
+++ b/extensions/cookie/nsPermissionManager.cpp
@@ -935,7 +935,7 @@ nsPermissionManager::InitDB(bool aRemoveFile)
       }
 
       // fall through to the next upgrade
-      MOZ_FALLTHROUGH;
+      [[fallthrough]];
 
     // TODO: we want to make default version as version 2 in order to fix bug 784875.
     case 0:
@@ -955,7 +955,7 @@ nsPermissionManager::InitDB(bool aRemoveFile)
       }
 
       // fall through to the next upgrade
-      MOZ_FALLTHROUGH;
+      [[fallthrough]];
 
     // Version 3->4 is the creation of the modificationTime field.
     case 3:
@@ -973,7 +973,7 @@ nsPermissionManager::InitDB(bool aRemoveFile)
       }
 
       // fall through to the next upgrade
-      MOZ_FALLTHROUGH;
+      [[fallthrough]];
 
     // In version 5, host appId, and isInBrowserElement were merged into a
     // single origin entry
@@ -1059,7 +1059,7 @@ nsPermissionManager::InitDB(bool aRemoveFile)
       }
 
       // fall through to the next upgrade
-      MOZ_FALLTHROUGH;
+      [[fallthrough]];
 
     // At this point, the version 5 table has been migrated to a version 6 table
     // We are guaranteed to have at least one of moz_hosts and moz_perms. If
@@ -1216,7 +1216,7 @@ nsPermissionManager::InitDB(bool aRemoveFile)
       }
 
       // fall through to the next upgrade
-      MOZ_FALLTHROUGH;
+      [[fallthrough]];
 
     // The version 7-8 migration is the re-migration of localhost and ip-address
     // entries due to errors in the previous version 7 migration which caused
@@ -1318,7 +1318,7 @@ nsPermissionManager::InitDB(bool aRemoveFile)
       }
 
       // fall through to the next upgrade
-      MOZ_FALLTHROUGH;
+      [[fallthrough]];
 
     // The version 8-9 migration removes the unnecessary backup moz-hosts database contents.
     // as the data no longer needs to be migrated
@@ -1346,7 +1346,7 @@ nsPermissionManager::InitDB(bool aRemoveFile)
       }
 
       // fall through to the next upgrade
-      MOZ_FALLTHROUGH;
+      [[fallthrough]];
 
     // current version.
     case HOSTS_SCHEMA_VERSION:
diff --git a/gfx/layers/apz/src/AsyncPanZoomController.cpp b/gfx/layers/apz/src/AsyncPanZoomController.cpp
index 4808daf49c..88a2a40362 100644
--- a/gfx/layers/apz/src/AsyncPanZoomController.cpp
+++ b/gfx/layers/apz/src/AsyncPanZoomController.cpp
@@ -1089,7 +1089,7 @@ nsEventStatus AsyncPanZoomController::OnTouchStart(const MultiTouchInput& aEvent
     case PAN_MOMENTUM:
       MOZ_ASSERT(GetCurrentTouchBlock());
       GetCurrentTouchBlock()->GetOverscrollHandoffChain()->CancelAnimations(ExcludeOverscroll);
-      MOZ_FALLTHROUGH;
+      [[fallthrough]];
     case NOTHING: {
       mX.StartTouch(point.x, aEvent.mTime);
       mY.StartTouch(point.y, aEvent.mTime);
@@ -1196,7 +1196,7 @@ nsEventStatus AsyncPanZoomController::OnTouchEnd(const MultiTouchInput& aEvent)
   case FLING:
     // Should never happen.
     NS_WARNING("Received impossible touch end in OnTouchEnd.");
-    MOZ_FALLTHROUGH;
+    [[fallthrough]];
   case ANIMATING_ZOOM:
   case SMOOTH_SCROLL:
   case NOTHING:
diff --git a/gfx/layers/apz/src/GestureEventListener.cpp b/gfx/layers/apz/src/GestureEventListener.cpp
index 94ece07ee1..8495252571 100644
--- a/gfx/layers/apz/src/GestureEventListener.cpp
+++ b/gfx/layers/apz/src/GestureEventListener.cpp
@@ -438,7 +438,7 @@ void GestureEventListener::HandleInputTimeoutLongTap()
     // just in case MAX_TAP_TIME > ContextMenuDelay cancel MAX_TAP timer
     // and fall through
     CancelMaxTapTimeoutTask();
-    MOZ_FALLTHROUGH;
+    [[fallthrough]];
   case GESTURE_FIRST_SINGLE_TOUCH_MAX_TAP_DOWN: {
     SetState(GESTURE_LONG_TOUCH_DOWN);
     mAsyncPanZoomController->HandleGestureEvent(
diff --git a/gfx/layers/apz/util/APZEventState.cpp b/gfx/layers/apz/util/APZEventState.cpp
index 00a18d7b26..d61ca3fd0c 100644
--- a/gfx/layers/apz/util/APZEventState.cpp
+++ b/gfx/layers/apz/util/APZEventState.cpp
@@ -347,10 +347,10 @@ APZEventState::ProcessTouchEvent(const WidgetTouchEvent& aEvent,
       mTouchEndCancelled = true;
       mEndTouchIsClick = false;
     }
-    MOZ_FALLTHROUGH;
+    [[fallthrough]];
   case eTouchCancel:
     mActiveElementManager->HandleTouchEndEvent(mEndTouchIsClick);
-    MOZ_FALLTHROUGH;
+    [[fallthrough]];
   case eTouchMove: {
     if (mPendingTouchPreventedResponse) {
       MOZ_ASSERT(aGuid == mPendingTouchPreventedGuid);
diff --git a/gfx/thebes/gfxFcPlatformFontList.cpp b/gfx/thebes/gfxFcPlatformFontList.cpp
index f80196430f..903be6adfb 100644
--- a/gfx/thebes/gfxFcPlatformFontList.cpp
+++ b/gfx/thebes/gfxFcPlatformFontList.cpp
@@ -625,7 +625,7 @@ PrepareFontOptions(FcPattern* aPattern,
             // subpixel_order won't be used by the font as we won't use
             // CAIRO_ANTIALIAS_SUBPIXEL, but don't leave it at default for
             // caching reasons described above.  Fall through:
-            MOZ_FALLTHROUGH;
+            [[fallthrough]];
         case FC_RGBA_RGB:
             subpixel_order = CAIRO_SUBPIXEL_ORDER_RGB;
             break;
diff --git a/gfx/thebes/gfxFont.cpp b/gfx/thebes/gfxFont.cpp
index 3857c4cd38..9eea084fd9 100644
--- a/gfx/thebes/gfxFont.cpp
+++ b/gfx/thebes/gfxFont.cpp
@@ -479,7 +479,7 @@ gfxFontShaper::MergeFontFeatures(
         case NS_FONT_VARIANT_CAPS_ALLSMALL:
             mergedFeatures.Put(HB_TAG('c','2','s','c'), 1);
             // fall through to the small-caps case
-            MOZ_FALLTHROUGH;
+            [[fallthrough]];
 
         case NS_FONT_VARIANT_CAPS_SMALLCAPS:
             mergedFeatures.Put(HB_TAG('s','m','c','p'), 1);
@@ -489,7 +489,7 @@ gfxFontShaper::MergeFontFeatures(
             mergedFeatures.Put(aAddSmallCaps ? HB_TAG('c','2','s','c') :
                                                HB_TAG('c','2','p','c'), 1);
             // fall through to the petite-caps case
-            MOZ_FALLTHROUGH;
+            [[fallthrough]];
 
         case NS_FONT_VARIANT_CAPS_PETITECAPS:
             mergedFeatures.Put(aAddSmallCaps ? HB_TAG('s','m','c','p') :
@@ -615,7 +615,7 @@ gfxFontShaper::GetRoundOffsetsToPixels(DrawTarget* aDrawTarget,
                     DWRITE_MEASURING_MODE_NATURAL) {
                 return;
             }
-            MOZ_FALLTHROUGH;
+            [[fallthrough]];
 #endif
         case CAIRO_FONT_TYPE_QUARTZ:
             // Quartz surfaces implement show_glyphs for Quartz fonts
@@ -3119,7 +3119,7 @@ gfxFont::InitFakeSmallCapsRun(DrawTarget     *aDrawTarget,
             case kUppercaseReduce:
                 // use reduced-size font, then fall through to uppercase the text
                 f = smallCapsFont;
-                MOZ_FALLTHROUGH;
+                [[fallthrough]];
 
             case kUppercase:
                 // apply uppercase transform to the string
diff --git a/gfx/thebes/gfxTextRun.cpp b/gfx/thebes/gfxTextRun.cpp
index 100af6da7b..896f2e1f8e 100644
--- a/gfx/thebes/gfxTextRun.cpp
+++ b/gfx/thebes/gfxTextRun.cpp
@@ -1758,7 +1758,7 @@ gfxFontGroup::FamilyFace::CheckState(bool& aSkipDrawing)
             case gfxUserFontEntry::STATUS_FAILED:
                 SetInvalid();
                 // fall-thru to the default case
-                MOZ_FALLTHROUGH;
+                [[fallthrough]];
             default:
                 SetLoading(false);
         }
diff --git a/image/decoders/nsJPEGDecoder.cpp b/image/decoders/nsJPEGDecoder.cpp
index 1ac25fc6e2..2fd38fc10b 100644
--- a/image/decoders/nsJPEGDecoder.cpp
+++ b/image/decoders/nsJPEGDecoder.cpp
@@ -412,7 +412,7 @@ nsJPEGDecoder::ReadJPEGData(const char* aData, size_t aLength)
             mInfo.output_width, mInfo.output_height));
 
     mState = JPEG_START_DECOMPRESS;
-    MOZ_FALLTHROUGH; // to start decompressing.
+    [[fallthrough]]; // to start decompressing.
   }
 
   case JPEG_START_DECOMPRESS: {
@@ -439,7 +439,7 @@ nsJPEGDecoder::ReadJPEGData(const char* aData, size_t aLength)
     // If this is a progressive JPEG ...
     mState = mInfo.buffered_image ?
              JPEG_DECOMPRESS_PROGRESSIVE : JPEG_DECOMPRESS_SEQUENTIAL;
-    MOZ_FALLTHROUGH; // to decompress sequential JPEG.
+    [[fallthrough]]; // to decompress sequential JPEG.
   }
 
   case JPEG_DECOMPRESS_SEQUENTIAL: {
@@ -461,7 +461,7 @@ nsJPEGDecoder::ReadJPEGData(const char* aData, size_t aLength)
                    "We didn't process all of the data!");
       mState = JPEG_DONE;
     }
-    MOZ_FALLTHROUGH; // to decompress progressive JPEG.
+    [[fallthrough]]; // to decompress progressive JPEG.
   }
 
   case JPEG_DECOMPRESS_PROGRESSIVE: {
@@ -534,7 +534,7 @@ nsJPEGDecoder::ReadJPEGData(const char* aData, size_t aLength)
 
       mState = JPEG_DONE;
     }
-    MOZ_FALLTHROUGH; // to finish decompressing.
+    [[fallthrough]]; // to finish decompressing.
   }
 
   case JPEG_DONE: {
diff --git a/intl/uconv/ucvja/nsJapaneseToUnicode.cpp b/intl/uconv/ucvja/nsJapaneseToUnicode.cpp
index 5bdfc47926..f33e60ede6 100644
--- a/intl/uconv/ucvja/nsJapaneseToUnicode.cpp
+++ b/intl/uconv/ucvja/nsJapaneseToUnicode.cpp
@@ -336,7 +336,7 @@ NS_IMETHODIMP nsEUCJPToUnicodeV2::Convert(
               break;
             }
             // else fall through to error handler
-            MOZ_FALLTHROUGH;
+            [[fallthrough]];
           }
           case 5: // two bytes undefined
           {
diff --git a/js/src/builtin/ReflectParse.cpp b/js/src/builtin/ReflectParse.cpp
index 7eb7c46b2d..e909528b5d 100644
--- a/js/src/builtin/ReflectParse.cpp
+++ b/js/src/builtin/ReflectParse.cpp
@@ -2537,7 +2537,7 @@ ASTSerializer::statement(ParseNode* pn, MutableHandleValue dst)
         pn = pn->as<LexicalScopeNode>().scopeBody();
         if (!pn->isKind(PNK_STATEMENTLIST))
             return statement(pn, dst);
-        MOZ_FALLTHROUGH;
+        [[fallthrough]];
 
       case PNK_STATEMENTLIST:
         return blockStatement(&pn->as<ListNode>(), dst);
diff --git a/js/src/ctypes/CTypes.cpp b/js/src/ctypes/CTypes.cpp
index 3bc249e015..26e8c63cfe 100644
--- a/js/src/ctypes/CTypes.cpp
+++ b/js/src/ctypes/CTypes.cpp
@@ -4519,7 +4519,7 @@ CType::Finalize(JSFreeOp* fop, JSObject* obj)
     }
   }
 
-    MOZ_FALLTHROUGH;
+    [[fallthrough]];
 
   case TYPE_array: {
     // Free the ffi_type info.
diff --git a/js/src/frontend/BytecodeEmitter.cpp b/js/src/frontend/BytecodeEmitter.cpp
index 89b768344a..99204fe9f2 100644
--- a/js/src/frontend/BytecodeEmitter.cpp
+++ b/js/src/frontend/BytecodeEmitter.cpp
@@ -1271,7 +1271,7 @@ BytecodeEmitter::checkSideEffects(ParseNode* pn, bool* answer)
       // Any subexpression of a comma expression could be effectful.
       case PNK_COMMA:
         MOZ_ASSERT(!pn->as<ListNode>().empty());
-        MOZ_FALLTHROUGH;
+        [[fallthrough]];
       // Subcomponents of a literal may be effectful.
       case PNK_ARRAY:
       case PNK_OBJECT:
diff --git a/js/src/frontend/NameFunctions.cpp b/js/src/frontend/NameFunctions.cpp
index 3fbdd3fc8a..351611e14f 100644
--- a/js/src/frontend/NameFunctions.cpp
+++ b/js/src/frontend/NameFunctions.cpp
@@ -174,7 +174,7 @@ class NameResolver
                  * flagged as a contributor.
                  */
                 pos--;
-                MOZ_FALLTHROUGH;
+                [[fallthrough]];
 
               default:
                 /* Save any other nodes we encounter on the way up. */
diff --git a/js/src/frontend/Parser.cpp b/js/src/frontend/Parser.cpp
index 96b83f7c48..037eb8dd07 100644
--- a/js/src/frontend/Parser.cpp
+++ b/js/src/frontend/Parser.cpp
@@ -1544,12 +1544,12 @@ Parser<ParseHandler>::noteDeclaredName(HandlePropertyName name, DeclarationKind
             return false;
         }
 
-        MOZ_FALLTHROUGH;
+        [[fallthrough]];
 
       case DeclarationKind::Import:
         // Module code is always strict, so 'let' is always a keyword and never a name.
         MOZ_ASSERT(name != context->names().let);
-        MOZ_FALLTHROUGH;
+        [[fallthrough]];
 
       case DeclarationKind::SimpleCatchParameter:
       case DeclarationKind::CatchParameter: {
@@ -6921,7 +6921,7 @@ Parser<ParseHandler>::yieldExpression(InHandling inHandling)
           case TOK_MUL:
             kind = PNK_YIELD_STAR;
             tokenStream.consumeKnownToken(TOK_MUL, TokenStream::Operand);
-            MOZ_FALLTHROUGH;
+            [[fallthrough]];
           default:
             exprNode = assignExpr(inHandling, YieldIsKeyword, TripledotProhibited);
             if (!exprNode)
@@ -6972,7 +6972,7 @@ Parser<ParseHandler>::yieldExpression(InHandling inHandling)
 
         pc->functionBox()->setGeneratorKind(LegacyGenerator);
 
-        MOZ_FALLTHROUGH;
+        [[fallthrough]];
 
       case LegacyGenerator:
       {
@@ -9618,7 +9618,7 @@ Parser<ParseHandler>::unaryExpr(YieldHandling yieldHandling, TripledotHandling t
         }
       }
 
-        MOZ_FALLTHROUGH;
+        [[fallthrough]];
 
       default: {
         Node expr = optionalExpr(yieldHandling, tripledotHandling, tt,
diff --git a/js/src/irregexp/RegExpEngine.cpp b/js/src/irregexp/RegExpEngine.cpp
index 75c6f4547a..0fb783170e 100644
--- a/js/src/irregexp/RegExpEngine.cpp
+++ b/js/src/irregexp/RegExpEngine.cpp
@@ -3348,7 +3348,7 @@ EmitAtomLetter(RegExpCompiler* compiler,
       }
       case 4:
         macro_assembler->CheckCharacter(chars[3], &ok);
-        MOZ_FALLTHROUGH;
+        [[fallthrough]];
       case 3:
         macro_assembler->CheckCharacter(chars[0], &ok);
         macro_assembler->CheckCharacter(chars[1], &ok);
diff --git a/js/src/irregexp/RegExpParser.cpp b/js/src/irregexp/RegExpParser.cpp
index 025efbb0ac..cd9277f818 100644
--- a/js/src/irregexp/RegExpParser.cpp
+++ b/js/src/irregexp/RegExpParser.cpp
@@ -987,7 +987,7 @@ RegExpParser<CharT>::ParseClassEscape(char16_t* char_class, widechar *value,
               }
               return true;
             }
-            MOZ_FALLTHROUGH;
+            [[fallthrough]];
           default:
             if (!ParseClassCharacterEscape(value))
                 return false;
@@ -1981,7 +1981,7 @@ RegExpParser<CharT>::ParseDisjunction()
                     Advance(2);
                     break;
                 }
-                MOZ_FALLTHROUGH;
+                [[fallthrough]];
               }
               case '0': {
                 if (unicode_) {
@@ -2094,7 +2094,7 @@ RegExpParser<CharT>::ParseDisjunction()
             int dummy;
             if (ParseIntervalQuantifier(&dummy, &dummy))
                 return ReportError(JSMSG_NOTHING_TO_REPEAT);
-            MOZ_FALLTHROUGH;
+            [[fallthrough]];
           }
           default:
             if (unicode_) {
diff --git a/js/src/jit/BacktrackingAllocator.cpp b/js/src/jit/BacktrackingAllocator.cpp
index 04078cb20c..af1a9789ac 100644
--- a/js/src/jit/BacktrackingAllocator.cpp
+++ b/js/src/jit/BacktrackingAllocator.cpp
@@ -2533,7 +2533,7 @@ BacktrackingAllocator::computeSpillWeight(LiveBundle* bundle)
 
               case LUse::FIXED:
                 fixed = true;
-                MOZ_FALLTHROUGH;
+                [[fallthrough]];
               case LUse::REGISTER:
                 usesTotal += 2000;
                 break;
diff --git a/js/src/jit/BaselineBailouts.cpp b/js/src/jit/BaselineBailouts.cpp
index 7b1bdc8c3f..22045b675f 100644
--- a/js/src/jit/BaselineBailouts.cpp
+++ b/js/src/jit/BaselineBailouts.cpp
@@ -1975,7 +1975,7 @@ jit::FinishBailoutToBaseline(BaselineBailoutInfo* bailoutInfo)
       // Invalid assumption based on baseline code.
       case Bailout_OverflowInvalidate:
         outerScript->setHadOverflowBailout();
-        MOZ_FALLTHROUGH;
+        [[fallthrough]];
       case Bailout_NonStringInputInvalidate:
       case Bailout_DoubleOutput:
       case Bailout_ObjectIdentityOrTypeGuard:
diff --git a/js/src/jit/BaselineIC.cpp b/js/src/jit/BaselineIC.cpp
index a6adc122c7..5e42dbc69a 100644
--- a/js/src/jit/BaselineIC.cpp
+++ b/js/src/jit/BaselineIC.cpp
@@ -929,7 +929,7 @@ RemoveExistingGetElemNativeStubs(JSContext* cx, ICGetElem_Fallback* stub, Handle
           case ICStub::GetElem_NativeSlotSymbol:
             if (indirect)
                 continue;
-            MOZ_FALLTHROUGH;
+            [[fallthrough]];
           case ICStub::GetElem_NativePrototypeSlotName:
           case ICStub::GetElem_NativePrototypeSlotSymbol:
           case ICStub::GetElem_NativePrototypeCallNativeName:
diff --git a/js/src/jit/IonBuilder.cpp b/js/src/jit/IonBuilder.cpp
index 7eb69ec00f..b04a0880a9 100644
--- a/js/src/jit/IonBuilder.cpp
+++ b/js/src/jit/IonBuilder.cpp
@@ -1596,7 +1596,7 @@ IonBuilder::traverseBytecode()
                 MOZ_ASSERT(i == 0);
                 if (current->peek(-1) == popped[0])
                     break;
-                MOZ_FALLTHROUGH;
+                [[fallthrough]];
 
               default:
                 MOZ_ASSERT(popped[i]->isImplicitlyUsed() ||
@@ -2032,7 +2032,7 @@ IonBuilder::inspectOpcode(JSOp op)
             }
         }
         // Fall through to JSOP_BINDNAME
-        MOZ_FALLTHROUGH;
+        [[fallthrough]];
       case JSOP_BINDNAME:
         return jsop_bindname(info().getName(pc));
 
diff --git a/js/src/jit/Lowering.cpp b/js/src/jit/Lowering.cpp
index 7f29d537a7..65895ca43d 100644
--- a/js/src/jit/Lowering.cpp
+++ b/js/src/jit/Lowering.cpp
@@ -2022,7 +2022,7 @@ LIRGenerator::visitToDouble(MToDouble* convert)
 
       case MIRType::Boolean:
         MOZ_ASSERT(conversion != MToFPInstruction::NumbersOnly);
-        MOZ_FALLTHROUGH;
+        [[fallthrough]];
 
       case MIRType::Int32:
       {
@@ -2077,7 +2077,7 @@ LIRGenerator::visitToFloat32(MToFloat32* convert)
 
       case MIRType::Boolean:
         MOZ_ASSERT(conversion != MToFPInstruction::NumbersOnly);
-        MOZ_FALLTHROUGH;
+        [[fallthrough]];
 
       case MIRType::Int32:
       {
diff --git a/js/src/jit/MCallOptimize.cpp b/js/src/jit/MCallOptimize.cpp
index 0f78ef3f2f..f4a1c0c199 100644
--- a/js/src/jit/MCallOptimize.cpp
+++ b/js/src/jit/MCallOptimize.cpp
@@ -2432,7 +2432,7 @@ IonBuilder::inlineIsTypedArrayHelper(CallInfo& callInfo, WrappingBehavior wrappi
             }
         }
 
-        MOZ_FALLTHROUGH;
+        [[fallthrough]];
 
       case TemporaryTypeSet::ForAllResult::EMPTY:
         result = false;
diff --git a/js/src/jit/MIR.cpp b/js/src/jit/MIR.cpp
index e7cb0e55f0..339ef9907b 100644
--- a/js/src/jit/MIR.cpp
+++ b/js/src/jit/MIR.cpp
@@ -2409,7 +2409,7 @@ CanProduceNegativeZero(MDefinition* def)
         case MDefinition::Op_Constant:
             if (def->type() == MIRType::Double && def->toConstant()->toDouble() == -0.0)
                 return true;
-            MOZ_FALLTHROUGH;
+            [[fallthrough]];
         case MDefinition::Op_BitAnd:
         case MDefinition::Op_BitOr:
         case MDefinition::Op_BitXor:
@@ -2491,7 +2491,7 @@ NeedNegativeZeroCheck(MDefinition* def)
             if (rhs->id() < lhs->id() && CanProduceNegativeZero(lhs))
                 return true;
 
-            MOZ_FALLTHROUGH;
+            [[fallthrough]];
           }
           case MDefinition::Op_StoreElement:
           case MDefinition::Op_StoreElementHole:
@@ -3379,7 +3379,7 @@ MTypeOf::foldsTo(TempAllocator& alloc)
             type = JSTYPE_OBJECT;
             break;
         }
-        MOZ_FALLTHROUGH;
+        [[fallthrough]];
       default:
         return this;
     }
diff --git a/js/src/jit/MacroAssembler.cpp b/js/src/jit/MacroAssembler.cpp
index 6721dd4419..3ff137631e 100644
--- a/js/src/jit/MacroAssembler.cpp
+++ b/js/src/jit/MacroAssembler.cpp
@@ -405,7 +405,7 @@ MacroAssembler::loadUnboxedProperty(T address, JSValueType type, TypedOrValueReg
               convertInt32ToDouble(address, output.typedReg().fpu());
               break;
           }
-          MOZ_FALLTHROUGH;
+          [[fallthrough]];
       }
 
       case JSVAL_TYPE_BOOLEAN:
diff --git a/js/src/jit/RangeAnalysis.cpp b/js/src/jit/RangeAnalysis.cpp
index bf1ea72d2a..09364e49f4 100644
--- a/js/src/jit/RangeAnalysis.cpp
+++ b/js/src/jit/RangeAnalysis.cpp
@@ -276,7 +276,7 @@ RangeAnalysis::addBetaNodes()
             if (!compare->isNumericComparison())
                 continue;
             // Otherwise fall through to handle JSOP_STRICTEQ the same as JSOP_EQ.
-            MOZ_FALLTHROUGH;
+            [[fallthrough]];
           case JSOP_EQ:
             comp.setDouble(bound, bound);
             break;
@@ -285,7 +285,7 @@ RangeAnalysis::addBetaNodes()
             if (!compare->isNumericComparison())
                 continue;
             // Otherwise fall through to handle JSOP_STRICTNE the same as JSOP_NE.
-            MOZ_FALLTHROUGH;
+            [[fallthrough]];
           case JSOP_NE:
             // Negative zero is not not-equal to zero.
             if (bound == 0) {
diff --git a/js/src/jit/arm/disasm/Disasm-arm.cpp b/js/src/jit/arm/disasm/Disasm-arm.cpp
index 678c8de89d..80f13fc1d8 100644
--- a/js/src/jit/arm/disasm/Disasm-arm.cpp
+++ b/js/src/jit/arm/disasm/Disasm-arm.cpp
@@ -1903,7 +1903,7 @@ Decoder::DecodeSpecialCondition(Instruction* instr)
             }
             break;
         }
-        MOZ_FALLTHROUGH;
+        [[fallthrough]];
       case 0xB:
         if ((instr->Bits(22, 20) == 5) && (instr->Bits(15, 12) == 0xf)) {
             int Rn = instr->Bits(19, 16);
diff --git a/js/src/jsdtoa.cpp b/js/src/jsdtoa.cpp
index e7ee040601..0414365e6a 100644
--- a/js/src/jsdtoa.cpp
+++ b/js/src/jsdtoa.cpp
@@ -146,7 +146,7 @@ js_dtostr(DtoaState* state, char* buffer, size_t bufferSize, JSDToStrMode mode,
             case DTOSTR_EXPONENTIAL:
                 MOZ_ASSERT(precision > 0);
                 minNDigits = precision;
-                MOZ_FALLTHROUGH;
+                [[fallthrough]];
             case DTOSTR_STANDARD_EXPONENTIAL:
                 exponentialNotation = true;
                 break;
diff --git a/js/src/jsgc.cpp b/js/src/jsgc.cpp
index c849bacc8c..a488740ee3 100644
--- a/js/src/jsgc.cpp
+++ b/js/src/jsgc.cpp
@@ -5418,7 +5418,7 @@ GCRuntime::incrementalCollectSlice(SliceBudget& budget, JS::gcreason::Reason rea
 
         incrementalState = State::MarkRoots;
 
-        MOZ_FALLTHROUGH;
+        [[fallthrough]];
 
       case State::MarkRoots:
         if (!beginMarkPhase(reason, lock)) {
@@ -5428,7 +5428,7 @@ GCRuntime::incrementalCollectSlice(SliceBudget& budget, JS::gcreason::Reason rea
 
         incrementalState = State::Mark;
 
-        MOZ_FALLTHROUGH;
+        [[fallthrough]];
 
       case State::Mark:
         AutoGCRooter::traceAllWrappers(&marker);
@@ -5472,7 +5472,7 @@ GCRuntime::incrementalCollectSlice(SliceBudget& budget, JS::gcreason::Reason rea
         if (budget.isOverBudget())
             break;
 
-        MOZ_FALLTHROUGH;
+        [[fallthrough]];
 
       case State::Sweep:
         if (performSweepActions(budget, lock) == NotFinished)
@@ -5482,7 +5482,7 @@ GCRuntime::incrementalCollectSlice(SliceBudget& budget, JS::gcreason::Reason rea
 
         incrementalState = State::Finalize;
 
-        MOZ_FALLTHROUGH;
+        [[fallthrough]];
 
       case State::Finalize:
         {
@@ -5516,7 +5516,7 @@ GCRuntime::incrementalCollectSlice(SliceBudget& budget, JS::gcreason::Reason rea
         if (isCompacting && isIncremental)
             break;
 
-        MOZ_FALLTHROUGH;
+        [[fallthrough]];
 
       case State::Compact:
         if (isCompacting) {
@@ -5532,7 +5532,7 @@ GCRuntime::incrementalCollectSlice(SliceBudget& budget, JS::gcreason::Reason rea
         startDecommit();
         incrementalState = State::Decommit;
 
-        MOZ_FALLTHROUGH;
+        [[fallthrough]];
 
       case State::Decommit:
         {
diff --git a/js/src/jsiter.cpp b/js/src/jsiter.cpp
index 726a84985a..9fcf449adb 100644
--- a/js/src/jsiter.cpp
+++ b/js/src/jsiter.cpp
@@ -523,13 +523,13 @@ Compare(T* a, T* b, size_t c)
 {
     size_t n = (c + size_t(7)) / size_t(8);
     switch (c % 8) {
-      case 0: do { if (*a++ != *b++) return false; MOZ_FALLTHROUGH;
-      case 7:      if (*a++ != *b++) return false; MOZ_FALLTHROUGH;
-      case 6:      if (*a++ != *b++) return false; MOZ_FALLTHROUGH;
-      case 5:      if (*a++ != *b++) return false; MOZ_FALLTHROUGH;
-      case 4:      if (*a++ != *b++) return false; MOZ_FALLTHROUGH;
-      case 3:      if (*a++ != *b++) return false; MOZ_FALLTHROUGH;
-      case 2:      if (*a++ != *b++) return false; MOZ_FALLTHROUGH;
+      case 0: do { if (*a++ != *b++) return false; [[fallthrough]];
+      case 7:      if (*a++ != *b++) return false; [[fallthrough]];
+      case 6:      if (*a++ != *b++) return false; [[fallthrough]];
+      case 5:      if (*a++ != *b++) return false; [[fallthrough]];
+      case 4:      if (*a++ != *b++) return false; [[fallthrough]];
+      case 3:      if (*a++ != *b++) return false; [[fallthrough]];
+      case 2:      if (*a++ != *b++) return false; [[fallthrough]];
       case 1:      if (*a++ != *b++) return false;
               } while (--n > 0);
     }
diff --git a/js/src/jsstr.cpp b/js/src/jsstr.cpp
index c539141130..f56fae9dd3 100644
--- a/js/src/jsstr.cpp
+++ b/js/src/jsstr.cpp
@@ -1613,13 +1613,13 @@ FirstCharMatcherUnrolled(const TextChar* text, uint32_t n, const PatChar pat)
     const TextChar* t = text;
 
     switch ((textend - t) & 7) {
-        case 0: if (*t++ == pat) return t - 1; MOZ_FALLTHROUGH;
-        case 7: if (*t++ == pat) return t - 1; MOZ_FALLTHROUGH;
-        case 6: if (*t++ == pat) return t - 1; MOZ_FALLTHROUGH;
-        case 5: if (*t++ == pat) return t - 1; MOZ_FALLTHROUGH;
-        case 4: if (*t++ == pat) return t - 1; MOZ_FALLTHROUGH;
-        case 3: if (*t++ == pat) return t - 1; MOZ_FALLTHROUGH;
-        case 2: if (*t++ == pat) return t - 1; MOZ_FALLTHROUGH;
+        case 0: if (*t++ == pat) return t - 1; [[fallthrough]];
+        case 7: if (*t++ == pat) return t - 1; [[fallthrough]];
+        case 6: if (*t++ == pat) return t - 1; [[fallthrough]];
+        case 5: if (*t++ == pat) return t - 1; [[fallthrough]];
+        case 4: if (*t++ == pat) return t - 1; [[fallthrough]];
+        case 3: if (*t++ == pat) return t - 1; [[fallthrough]];
+        case 2: if (*t++ == pat) return t - 1; [[fallthrough]];
         case 1: if (*t++ == pat) return t - 1;
     }
     while (textend != t) {
diff --git a/js/src/shell/js.cpp b/js/src/shell/js.cpp
index b83fa6a3d5..a90a1a7198 100644
--- a/js/src/shell/js.cpp
+++ b/js/src/shell/js.cpp
@@ -1548,7 +1548,7 @@ ConvertTranscodeResultToJSException(JSContext* cx, JS::TranscodeResult rv)
         return true;
 
       default:
-        MOZ_FALLTHROUGH;
+        [[fallthrough]];
       case JS::TranscodeResult_Failure:
         MOZ_ASSERT(!cx->isExceptionPending());
         JS_ReportErrorASCII(cx, "generic warning");
diff --git a/js/src/vm/RegExpObject.cpp b/js/src/vm/RegExpObject.cpp
index 1d88754084..88a4ed72e2 100644
--- a/js/src/vm/RegExpObject.cpp
+++ b/js/src/vm/RegExpObject.cpp
@@ -1513,7 +1513,7 @@ ParseRegExpFlags(const CharT* chars, size_t length, RegExpFlag* flagsOut, char16
                     return false;
                 break;
             }
-            MOZ_FALLTHROUGH;
+            [[fallthrough]];
           default:
             return false;
         }
diff --git a/js/src/wasm/WasmBaselineCompile.cpp b/js/src/wasm/WasmBaselineCompile.cpp
index 38d8d5b99b..850994b3f0 100644
--- a/js/src/wasm/WasmBaselineCompile.cpp
+++ b/js/src/wasm/WasmBaselineCompile.cpp
@@ -3301,7 +3301,7 @@ class BaseCompiler
           case Scalar::Uint16:
           case Scalar::Uint32: {
             isSigned = false;
-            MOZ_FALLTHROUGH;
+            [[fallthrough]];
           case Scalar::Int8:
           case Scalar::Int16:
           case Scalar::Int32:
@@ -3380,15 +3380,15 @@ class BaseCompiler
 
         switch (access.type()) {
           case Scalar::Uint8:
-            MOZ_FALLTHROUGH;
+            [[fallthrough]];
           case Scalar::Uint16:
-            MOZ_FALLTHROUGH;
+            [[fallthrough]];
           case Scalar::Int8:
-            MOZ_FALLTHROUGH;
+            [[fallthrough]];
           case Scalar::Int16:
-            MOZ_FALLTHROUGH;
+            [[fallthrough]];
           case Scalar::Int32:
-            MOZ_FALLTHROUGH;
+            [[fallthrough]];
           case Scalar::Uint32: {
             Register rt = src.tag == AnyReg::I64 ? src.i64().reg.low : src.i32().reg;
             storeI32(access, ptr, rt);
diff --git a/js/src/wasm/WasmTextToBinary.cpp b/js/src/wasm/WasmTextToBinary.cpp
index c3a707b550..66b97d380a 100644
--- a/js/src/wasm/WasmTextToBinary.cpp
+++ b/js/src/wasm/WasmTextToBinary.cpp
@@ -824,7 +824,7 @@ WasmTokenStream::next()
             return nan(begin);
         if (!IsWasmDigit(*cur_))
             break;
-        MOZ_FALLTHROUGH;
+        [[fallthrough]];
       case '0': case '1': case '2': case '3': case '4':
       case '5': case '6': case '7': case '8': case '9':
         return literal(begin);
diff --git a/js/xpconnect/src/XPCConvert.cpp b/js/xpconnect/src/XPCConvert.cpp
index 87876f36fb..74a7331e75 100644
--- a/js/xpconnect/src/XPCConvert.cpp
+++ b/js/xpconnect/src/XPCConvert.cpp
@@ -495,7 +495,7 @@ XPCConvert::JSData2Native(void* d, HandleValue s,
             (**((nsAString**)d)).SetIsVoid(true);
             return true;
         }
-        MOZ_FALLTHROUGH;
+        [[fallthrough]];
     }
     case nsXPTType::T_DOMSTRING:
     {
diff --git a/js/xpconnect/src/XPCShellImpl.cpp b/js/xpconnect/src/XPCShellImpl.cpp
index ba56a4a0ee..7f748c8f53 100644
--- a/js/xpconnect/src/XPCShellImpl.cpp
+++ b/js/xpconnect/src/XPCShellImpl.cpp
@@ -935,7 +935,7 @@ ProcessArgsForCompartment(JSContext* cx, char** argv, int argc)
             break;
         case 'S':
             ContextOptionsRef(cx).toggleWerror();
-            MOZ_FALLTHROUGH; // because -S implies -s
+            [[fallthrough]]; // because -S implies -s
         case 's':
             ContextOptionsRef(cx).toggleExtraWarnings();
             break;
diff --git a/layout/base/TouchManager.cpp b/layout/base/TouchManager.cpp
index 4439a3a7b0..0ca58b4360 100644
--- a/layout/base/TouchManager.cpp
+++ b/layout/base/TouchManager.cpp
@@ -213,7 +213,7 @@ TouchManager::PreHandleEvent(WidgetEvent* aEvent,
     case eTouchEnd:
       aIsHandlingUserInput = true;
       // Fall through to touchcancel code
-      MOZ_FALLTHROUGH;
+      [[fallthrough]];
     case eTouchCancel: {
       // Remove the changed touches
       // need to make sure we only remove touches that are ending here
diff --git a/layout/base/nsCSSRendering.cpp b/layout/base/nsCSSRendering.cpp
index 3084cf9085..74d0cabc6d 100644
--- a/layout/base/nsCSSRendering.cpp
+++ b/layout/base/nsCSSRendering.cpp
@@ -4404,7 +4404,7 @@ nsCSSRendering::DrawTableBorderSegment(DrawTarget&              aDrawTarget,
     break;
   case NS_STYLE_BORDER_STYLE_GROOVE:
     ridgeGroove = NS_STYLE_BORDER_STYLE_GROOVE; // and fall through to ridge
-    MOZ_FALLTHROUGH;
+    [[fallthrough]];
   case NS_STYLE_BORDER_STYLE_RIDGE:
     if ((horizontal && (twipsPerPixel >= aBorder.height)) ||
         (!horizontal && (twipsPerPixel >= aBorder.width))) {
@@ -4571,7 +4571,7 @@ nsCSSRendering::DrawTableBorderSegment(DrawTarget&              aDrawTarget,
       break;
     }
     // else fall through to solid
-    MOZ_FALLTHROUGH;
+    [[fallthrough]];
   case NS_STYLE_BORDER_STYLE_SOLID:
     DrawSolidBorderSegment(aDrawTarget, aBorder, aBorderColor,
                            aAppUnitsPerDevPixel, twipsPerPixel, aStartBevelSide,
diff --git a/layout/base/nsCSSRenderingBorders.cpp b/layout/base/nsCSSRenderingBorders.cpp
index 9720bb2dbf..5ca72075c8 100644
--- a/layout/base/nsCSSRenderingBorders.cpp
+++ b/layout/base/nsCSSRenderingBorders.cpp
@@ -1235,7 +1235,7 @@ MakeBorderColor(nscolor aColor, nscolor aBackgroundColor,
 
     case BorderColorStyleLight:
       k = 1;
-      MOZ_FALLTHROUGH;
+      [[fallthrough]];
     case BorderColorStyleDark:
       NS_GetSpecial3DColors(colors, aBackgroundColor, aColor);
       return Color::FromABGR(colors[k]);
diff --git a/layout/generic/CSSAlignUtils.cpp b/layout/generic/CSSAlignUtils.cpp
index 4cbaccd52c..ec993faf1a 100644
--- a/layout/generic/CSSAlignUtils.cpp
+++ b/layout/generic/CSSAlignUtils.cpp
@@ -133,7 +133,7 @@ CSSAlignUtils::AlignJustifySelf(uint8_t aAlignment, LogicalAxis aAxis,
       }
       break;
     case NS_STYLE_ALIGN_STRETCH:
-      MOZ_FALLTHROUGH; // ComputeSize() deals with it
+      [[fallthrough]]; // ComputeSize() deals with it
     case NS_STYLE_ALIGN_START:
       offset = marginStart;
       break;
diff --git a/layout/generic/nsFlexContainerFrame.cpp b/layout/generic/nsFlexContainerFrame.cpp
index d60308f424..2fc90e9da9 100644
--- a/layout/generic/nsFlexContainerFrame.cpp
+++ b/layout/generic/nsFlexContainerFrame.cpp
@@ -3012,7 +3012,7 @@ MainAxisPositionTracker::
       case NS_STYLE_JUSTIFY_BASELINE:
       case NS_STYLE_JUSTIFY_LAST_BASELINE:
         NS_WARNING("NYI: justify-content:left/right/baseline/last baseline");
-        MOZ_FALLTHROUGH;
+        [[fallthrough]];
       case NS_STYLE_JUSTIFY_FLEX_START:
         // All packing space should go at the end --> nothing to do here.
         break;
@@ -3220,7 +3220,7 @@ CrossAxisPositionTracker::
       case NS_STYLE_ALIGN_BASELINE:
       case NS_STYLE_ALIGN_LAST_BASELINE:
         NS_WARNING("NYI: align-items/align-self:left/right/self-start/self-end/baseline/last baseline");
-        MOZ_FALLTHROUGH;
+        [[fallthrough]];
       case NS_STYLE_ALIGN_FLEX_START:
         // All packing space should go at the end --> nothing to do here.
         break;
@@ -3522,7 +3522,7 @@ SingleLineCrossAxisPositionTracker::
     case NS_STYLE_ALIGN_SELF_START:
     case NS_STYLE_ALIGN_SELF_END:
       NS_WARNING("NYI: align-items/align-self:left/right/self-start/self-end");
-      MOZ_FALLTHROUGH;
+      [[fallthrough]];
     case NS_STYLE_ALIGN_FLEX_START:
       // No space to skip over -- we're done.
       break;
diff --git a/layout/generic/nsFrame.cpp b/layout/generic/nsFrame.cpp
index a4d4427f69..0d00a35865 100644
--- a/layout/generic/nsFrame.cpp
+++ b/layout/generic/nsFrame.cpp
@@ -7735,7 +7735,7 @@ nsIFrame::PeekOffset(nsPeekOffsetStruct* aPos)
         aPos->mWordMovementType = eEndWord;
       }
       // Intentionally fall through the eSelectWord case.
-      MOZ_FALLTHROUGH;
+      [[fallthrough]];
     case eSelectWord:
     {
       // wordSelectEatSpace means "are we looking for a boundary between whitespace
diff --git a/layout/generic/nsGridContainerFrame.cpp b/layout/generic/nsGridContainerFrame.cpp
index 1e1afac234..5cc8189e94 100644
--- a/layout/generic/nsGridContainerFrame.cpp
+++ b/layout/generic/nsGridContainerFrame.cpp
@@ -4938,7 +4938,7 @@ nsGridContainerFrame::Tracks::AlignJustifyContent(
     case NS_STYLE_ALIGN_BASELINE:
     case NS_STYLE_ALIGN_LAST_BASELINE:
       NS_WARNING("NYI: 'first/last baseline' (bug 1151204)"); // XXX
-      MOZ_FALLTHROUGH;
+      [[fallthrough]];
     case NS_STYLE_ALIGN_START:
       distribute = false;
       break;
diff --git a/layout/generic/nsLineLayout.cpp b/layout/generic/nsLineLayout.cpp
index 579c4ae0b0..90ce6b089f 100644
--- a/layout/generic/nsLineLayout.cpp
+++ b/layout/generic/nsLineLayout.cpp
@@ -3009,7 +3009,7 @@ nsLineLayout::ExpandRubyBox(PerFrameData* aFrame, nscoord aReservedISize,
       }
       // If there are no justification opportunities for space-between,
       // fall-through to center per spec.
-      MOZ_FALLTHROUGH;
+      [[fallthrough]];
     }
     case NS_STYLE_RUBY_ALIGN_CENTER:
       // Indent all children by half of the reserved inline size.
@@ -3201,7 +3201,7 @@ nsLineLayout::TextAlignLine(nsLineBox* aLine,
         }
         // Fall through to the default case if we could not justify to fill
         // the space.
-        MOZ_FALLTHROUGH;
+        [[fallthrough]];
       }
 
       case NS_STYLE_TEXT_ALIGN_START:
diff --git a/layout/printing/nsPrintPreviewListener.cpp b/layout/printing/nsPrintPreviewListener.cpp
index 5edc0fb903..f4344b3037 100644
--- a/layout/printing/nsPrintPreviewListener.cpp
+++ b/layout/printing/nsPrintPreviewListener.cpp
@@ -195,7 +195,7 @@ nsPrintPreviewListener::HandleEvent(nsIDOMEvent* aEvent)
           }
         }
       }
-      MOZ_FALLTHROUGH;
+      [[fallthrough]];
       case eEventAction_Suppress:
         aEvent->StopPropagation();
         aEvent->PreventDefault();
diff --git a/layout/style/CSSLexer.cpp b/layout/style/CSSLexer.cpp
index f465981e7b..a38784a192 100644
--- a/layout/style/CSSLexer.cpp
+++ b/layout/style/CSSLexer.cpp
@@ -125,7 +125,7 @@ CSSLexer::NextToken(Nullable<CSSToken>& aResult)
 
     case eCSSToken_Dimension:
       resultToken.mText.Construct(token.mIdent);
-      MOZ_FALLTHROUGH;
+      [[fallthrough]];
     case eCSSToken_Number:
     case eCSSToken_Percentage:
       resultToken.mNumber.Construct(token.mNumber);
diff --git a/layout/style/Declaration.cpp b/layout/style/Declaration.cpp
index 6ef53f5db1..9a196ef27b 100644
--- a/layout/style/Declaration.cpp
+++ b/layout/style/Declaration.cpp
@@ -770,7 +770,7 @@ Declaration::GetPropertyValueInternal(
       }
       // tweak aProperty and fall through
       aProperty = eCSSProperty_border_top;
-      MOZ_FALLTHROUGH;
+      [[fallthrough]];
     }
     case eCSSProperty_border_top:
     case eCSSProperty_border_right:
@@ -1321,7 +1321,7 @@ Declaration::GetPropertyValueInternal(
         return;
       }
       // Fall through to eCSSProperty_grid_template (syntax #1)
-      MOZ_FALLTHROUGH;
+      [[fallthrough]];
     }
     case eCSSProperty_grid_template: {
       const nsCSSValue& areasValue =
@@ -1466,7 +1466,7 @@ Declaration::GetPropertyValueInternal(
           !justify || !IsSingleValue(*justify)) {
         return; // Not serializable, bail.
       }
-      MOZ_FALLTHROUGH;
+      [[fallthrough]];
     }
     case eCSSProperty_gap: {
       const nsCSSPropertyID* subprops =
diff --git a/layout/style/StyleAnimationValue.cpp b/layout/style/StyleAnimationValue.cpp
index 2332de6a6a..199bcf16d5 100644
--- a/layout/style/StyleAnimationValue.cpp
+++ b/layout/style/StyleAnimationValue.cpp
@@ -164,7 +164,7 @@ AppendFunction(nsCSSKeyword aTransformFunction)
       break;
     default:
       NS_ERROR("must be a transform function");
-      MOZ_FALLTHROUGH;
+      [[fallthrough]];
     case eCSSKeyword_translatex:
     case eCSSKeyword_translatey:
     case eCSSKeyword_translatez:
@@ -2005,7 +2005,7 @@ AddWeightedFilterFunctionImpl(double aCoeff1, const nsCSSValueList* aList1,
     case eCSSKeyword_invert:
     case eCSSKeyword_sepia:
       initialVal = 0.0f;
-      MOZ_FALLTHROUGH;
+      [[fallthrough]];
     case eCSSKeyword_brightness:
     case eCSSKeyword_contrast:
     case eCSSKeyword_opacity:
@@ -2255,7 +2255,7 @@ AddShapeFunction(nsCSSPropertyID aProperty,
                                        resultFuncArgs->Item(2))) {
         return nullptr;
       }
-      MOZ_FALLTHROUGH;  // to handle rx and center point
+      [[fallthrough]];  // to handle rx and center point
     case eCSSKeyword_circle: {
       // Add circles' |r| (or ellipses' |rx|) values:
       if (!AddCSSValuePixelPercentCalc(aRestriction == Restrictions::Enable
@@ -2471,7 +2471,7 @@ AddTransformLists(double aCoeff1, const nsCSSValueList* aList1,
                            arr->Item(4));
           break;
         }
-        MOZ_FALLTHROUGH;
+        [[fallthrough]];
       }
       case eCSSKeyword_matrix:
       case eCSSKeyword_matrix3d:
@@ -2496,7 +2496,7 @@ AddTransformLists(double aCoeff1, const nsCSSValueList* aList1,
           }
           break;
         }
-        MOZ_FALLTHROUGH;
+        [[fallthrough]];
       case eCSSKeyword_interpolatematrix: {
         // FIXME: If the matrix contains only numbers then we could decompose
         // here.
diff --git a/layout/style/nsCSSParser.cpp b/layout/style/nsCSSParser.cpp
index f66415ea9e..2469536ad5 100644
--- a/layout/style/nsCSSParser.cpp
+++ b/layout/style/nsCSSParser.cpp
@@ -4580,7 +4580,7 @@ CSSParserImpl::ParseKeyframeSelectorList(InfallibleTArray<float>& aSelectorList)
           value = 1.0f;
           break;
         }
-        MOZ_FALLTHROUGH;
+        [[fallthrough]];
       default:
         UngetToken();
         // The first time through the loop, this means we got an empty
@@ -10759,7 +10759,7 @@ CSSParserImpl::IsLegacyGradientLine(const nsCSSTokenType& aType,
       haveGradientLine = true;
       break;
     }
-    MOZ_FALLTHROUGH;
+    [[fallthrough]];
   case eCSSToken_ID:
   case eCSSToken_Hash:
     // this is a color
@@ -11442,10 +11442,10 @@ CSSParserImpl::ParseBoxProperties(const nsCSSPropertyID aPropIDs[])
   switch (count) {
     case 1: // Make right == top
       result.mRight = result.mTop;
-      MOZ_FALLTHROUGH;
+      [[fallthrough]];
     case 2: // Make bottom == top
       result.mBottom = result.mTop;
-      MOZ_FALLTHROUGH;
+      [[fallthrough]];
     case 3: // Make left == right
       result.mLeft = result.mRight;
   }
@@ -11488,10 +11488,10 @@ CSSParserImpl::ParseGroupedBoxProperty(int32_t aVariantMask,
   switch (count) {
     case 1: // Make right == top
       result.mRight = result.mTop;
-      MOZ_FALLTHROUGH;
+      [[fallthrough]];
     case 2: // Make bottom == top
       result.mBottom = result.mTop;
-      MOZ_FALLTHROUGH;
+      [[fallthrough]];
     case 3: // Make left == right
       result.mLeft = result.mRight;
   }
@@ -11589,10 +11589,10 @@ CSSParserImpl::ParseBoxCornerRadiiInternals(nsCSSValue array[])
   switch (countX) {
     case 1: // Make top-right same as top-left
       dimenX.mRight = dimenX.mTop;
-      MOZ_FALLTHROUGH;
+      [[fallthrough]];
     case 2: // Make bottom-right same as top-left
       dimenX.mBottom = dimenX.mTop;
-      MOZ_FALLTHROUGH;
+      [[fallthrough]];
     case 3: // Make bottom-left same as top-right
       dimenX.mLeft = dimenX.mRight;
   }
@@ -11600,10 +11600,10 @@ CSSParserImpl::ParseBoxCornerRadiiInternals(nsCSSValue array[])
   switch (countY) {
     case 1: // Make top-right same as top-left
       dimenY.mRight = dimenY.mTop;
-      MOZ_FALLTHROUGH;
+      [[fallthrough]];
     case 2: // Make bottom-right same as top-left
       dimenY.mBottom = dimenY.mTop;
-      MOZ_FALLTHROUGH;
+      [[fallthrough]];
     case 3: // Make bottom-left same as top-right
       dimenY.mLeft = dimenY.mRight;
   }
diff --git a/layout/style/nsComputedDOMStyle.cpp b/layout/style/nsComputedDOMStyle.cpp
index 65f42805de..e3f931ea7e 100644
--- a/layout/style/nsComputedDOMStyle.cpp
+++ b/layout/style/nsComputedDOMStyle.cpp
@@ -4109,7 +4109,7 @@ nsComputedDOMStyle::DoGetTextSizeAdjust()
   switch (StyleText()->mTextSizeAdjust) {
     default:
       NS_NOTREACHED("unexpected value");
-      MOZ_FALLTHROUGH;
+      [[fallthrough]];
     case NS_STYLE_TEXT_SIZE_ADJUST_AUTO:
       val->SetIdent(eCSSKeyword_auto);
       break;
diff --git a/layout/style/nsRuleNode.cpp b/layout/style/nsRuleNode.cpp
index 858d7feba3..3eea109db6 100644
--- a/layout/style/nsRuleNode.cpp
+++ b/layout/style/nsRuleNode.cpp
@@ -240,7 +240,7 @@ nsRuleNode::EnsureBlockDisplay(StyleDisplay& display,
       display = StyleDisplay::Block;
       break;
     } // else, fall through to share the 'break' for non-changing display vals
-    MOZ_FALLTHROUGH;
+    [[fallthrough]];
   case StyleDisplay::None:
   case StyleDisplay::Contents:
     // never change display:none or display:contents *ever*
diff --git a/layout/style/nsStyleStruct.cpp b/layout/style/nsStyleStruct.cpp
index e83a6dcf95..f6304fd833 100644
--- a/layout/style/nsStyleStruct.cpp
+++ b/layout/style/nsStyleStruct.cpp
@@ -1287,7 +1287,7 @@ nsStyleSVGPaint::Reset()
     case eStyleSVGPaintType_Server:
       mPaint.mPaintServer->Release();
       mPaint.mPaintServer = nullptr;
-      MOZ_FALLTHROUGH;
+      [[fallthrough]];
     case eStyleSVGPaintType_ContextFill:
     case eStyleSVGPaintType_ContextStroke:
       mFallbackColor = NS_RGB(0, 0, 0);
diff --git a/layout/style/nsStyleTransformMatrix.cpp b/layout/style/nsStyleTransformMatrix.cpp
index 3cc1667074..3ac6b6b478 100644
--- a/layout/style/nsStyleTransformMatrix.cpp
+++ b/layout/style/nsStyleTransformMatrix.cpp
@@ -639,7 +639,7 @@ MatrixForTransformFunction(Matrix4x4& aMatrix,
     break;
   case eCSSKeyword_rotatez:
     *aContains3dTransform = true;
-    MOZ_FALLTHROUGH;
+    [[fallthrough]];
   case eCSSKeyword_rotate:
     ProcessRotateZ(aMatrix, aData);
     break;
diff --git a/layout/tables/SpanningCellSorter.cpp b/layout/tables/SpanningCellSorter.cpp
index 9ca577cde2..13aa61f939 100644
--- a/layout/tables/SpanningCellSorter.cpp
+++ b/layout/tables/SpanningCellSorter.cpp
@@ -108,7 +108,7 @@ SpanningCellSorter::GetNext(int32_t *aColSpan)
             /* prepare to enumerate the array */
             mState = ENUMERATING_ARRAY;
             mEnumerationIndex = 0;
-            MOZ_FALLTHROUGH;
+            [[fallthrough]];
         case ENUMERATING_ARRAY:
             while (mEnumerationIndex < ARRAY_SIZE && !mArray[mEnumerationIndex])
                 ++mEnumerationIndex;
@@ -138,7 +138,7 @@ SpanningCellSorter::GetNext(int32_t *aColSpan)
                              SortArray, nullptr);
                 mSortedHashTable = sh;
             }
-            MOZ_FALLTHROUGH;
+            [[fallthrough]];
         case ENUMERATING_HASH:
             if (mEnumerationIndex < mHashTable.EntryCount()) {
                 Item *result = mSortedHashTable[mEnumerationIndex]->mItems;
@@ -153,7 +153,7 @@ SpanningCellSorter::GetNext(int32_t *aColSpan)
                 return result;
             }
             mState = DONE;
-            MOZ_FALLTHROUGH;
+            [[fallthrough]];
         case DONE:
             ;
     }
diff --git a/layout/tables/nsCellMap.cpp b/layout/tables/nsCellMap.cpp
index 4852a4bdde..e0deda7824 100644
--- a/layout/tables/nsCellMap.cpp
+++ b/layout/tables/nsCellMap.cpp
@@ -913,7 +913,7 @@ nsTableCellMap::ResetBStartStart(LogicalSide aSide,
   switch(aSide) {
   case eLogicalSideBEnd:
     aRowIndex++;
-    MOZ_FALLTHROUGH;
+    [[fallthrough]];
   case eLogicalSideBStart:
     cellData = (BCCellData*)aCellMap.GetDataAt(aRowIndex, aColIndex);
     if (cellData) {
@@ -936,7 +936,7 @@ nsTableCellMap::ResetBStartStart(LogicalSide aSide,
     break;
   case eLogicalSideIEnd:
     aColIndex++;
-    MOZ_FALLTHROUGH;
+    [[fallthrough]];
   case eLogicalSideIStart:
     cellData = (BCCellData*)aCellMap.GetDataAt(aRowIndex, aColIndex);
     if (cellData) {
@@ -981,7 +981,7 @@ nsTableCellMap::SetBCBorderEdge(LogicalSide aSide,
   case eLogicalSideBEnd:
     rgYPos++;
     yPos++;
-    MOZ_FALLTHROUGH;
+    [[fallthrough]];
   case eLogicalSideBStart:
     lastIndex = xPos + aLength - 1;
     for (xIndex = xPos; xIndex <= lastIndex; xIndex++) {
@@ -1028,7 +1028,7 @@ nsTableCellMap::SetBCBorderEdge(LogicalSide aSide,
     break;
   case eLogicalSideIEnd:
     xPos++;
-    MOZ_FALLTHROUGH;
+    [[fallthrough]];
   case eLogicalSideIStart:
     // since bStart, bEnd borders were set, there should already be a cellData entry
     lastIndex = rgYPos + aLength - 1;
diff --git a/layout/tables/nsTableFrame.cpp b/layout/tables/nsTableFrame.cpp
index cf4231c01f..f6d2dd54b9 100644
--- a/layout/tables/nsTableFrame.cpp
+++ b/layout/tables/nsTableFrame.cpp
@@ -6893,7 +6893,7 @@ BCBlockDirSeg::Paint(BCPaintBorderIterator& aIter,
       if (!aIter.IsTableIEndMost() && (relColIndex > 0)) {
         col = aIter.mBlockDirInfo[relColIndex - 1].mCol;
       }
-      MOZ_FALLTHROUGH;
+      [[fallthrough]];
     case eColGroupOwner:
       if (col) {
         owner = col->GetParent();
@@ -6904,13 +6904,13 @@ BCBlockDirSeg::Paint(BCPaintBorderIterator& aIter,
       if (!aIter.IsTableIEndMost() && (relColIndex > 0)) {
         col = aIter.mBlockDirInfo[relColIndex - 1].mCol;
       }
-      MOZ_FALLTHROUGH;
+      [[fallthrough]];
     case eColOwner:
       owner = col;
       break;
     case eAjaRowGroupOwner:
       NS_ERROR("a neighboring rowgroup can never own a vertical border");
-      MOZ_FALLTHROUGH;
+      [[fallthrough]];
     case eRowGroupOwner:
       NS_ASSERTION(aIter.IsTableIStartMost() || aIter.IsTableIEndMost(),
                   "row group can own border only at table edge");
@@ -6918,7 +6918,7 @@ BCBlockDirSeg::Paint(BCPaintBorderIterator& aIter,
       break;
     case eAjaRowOwner:
       NS_ERROR("program error");
-      MOZ_FALLTHROUGH;
+      [[fallthrough]];
     case eRowOwner:
       NS_ASSERTION(aIter.IsTableIStartMost() || aIter.IsTableIEndMost(),
                    "row can own border only at table edge");
@@ -6927,7 +6927,7 @@ BCBlockDirSeg::Paint(BCPaintBorderIterator& aIter,
     case eAjaCellOwner:
       side = eLogicalSideIEnd;
       cell = mAjaCell;
-      MOZ_FALLTHROUGH;
+      [[fallthrough]];
     case eCellOwner:
       owner = cell;
       break;
@@ -7104,7 +7104,7 @@ BCInlineDirSeg::Paint(BCPaintBorderIterator& aIter, DrawTarget& aDrawTarget)
       break;
     case eAjaColGroupOwner:
       NS_ERROR("neighboring colgroups can never own an inline-dir border");
-      MOZ_FALLTHROUGH;
+      [[fallthrough]];
     case eColGroupOwner:
       NS_ASSERTION(aIter.IsTableBStartMost() || aIter.IsTableBEndMost(),
                    "col group can own border only at the table edge");
@@ -7114,7 +7114,7 @@ BCInlineDirSeg::Paint(BCPaintBorderIterator& aIter, DrawTarget& aDrawTarget)
       break;
     case eAjaColOwner:
       NS_ERROR("neighboring column can never own an inline-dir border");
-      MOZ_FALLTHROUGH;
+      [[fallthrough]];
     case eColOwner:
       NS_ASSERTION(aIter.IsTableBStartMost() || aIter.IsTableBEndMost(),
                    "col can own border only at the table edge");
@@ -7123,14 +7123,14 @@ BCInlineDirSeg::Paint(BCPaintBorderIterator& aIter, DrawTarget& aDrawTarget)
     case eAjaRowGroupOwner:
       side = eLogicalSideBEnd;
       rg = (aIter.IsTableBEndMost()) ? aIter.mRg : aIter.mPrevRg;
-      MOZ_FALLTHROUGH;
+      [[fallthrough]];
     case eRowGroupOwner:
       owner = rg;
       break;
     case eAjaRowOwner:
       side = eLogicalSideBEnd;
       row = (aIter.IsTableBEndMost()) ? aIter.mRow : aIter.mPrevRow;
-      MOZ_FALLTHROUGH;
+      [[fallthrough]];
     case eRowOwner:
       owner = row;
       break;
@@ -7139,7 +7139,7 @@ BCInlineDirSeg::Paint(BCPaintBorderIterator& aIter, DrawTarget& aDrawTarget)
       // if this is null due to the damage area origin-y > 0, then the border
       // won't show up anyway
       cell = mAjaCell;
-      MOZ_FALLTHROUGH;
+      [[fallthrough]];
     case eCellOwner:
       owner = cell;
       break;
diff --git a/layout/tables/nsTableRowFrame.cpp b/layout/tables/nsTableRowFrame.cpp
index 374bb2562e..9bcced1048 100644
--- a/layout/tables/nsTableRowFrame.cpp
+++ b/layout/tables/nsTableRowFrame.cpp
@@ -653,7 +653,7 @@ nsTableRowFrame::CalculateCellActualBSize(nsTableCellFrame* aCellFrame,
         break;
       }
       // Fall through to the coord case
-      MOZ_FALLTHROUGH;
+      [[fallthrough]];
     }
     case eStyleUnit_Coord: {
       // In quirks mode, table cell isize should be content-box, but bsize
diff --git a/layout/xul/nsXULPopupManager.cpp b/layout/xul/nsXULPopupManager.cpp
index 6e8cc3dda1..99cbd421dd 100644
--- a/layout/xul/nsXULPopupManager.cpp
+++ b/layout/xul/nsXULPopupManager.cpp
@@ -2336,7 +2336,7 @@ nsXULPopupManager::HandleKeyboardEventWithKeyCode(
         Rollup(0, false, nullptr, nullptr);
         break;
       }
-      MOZ_FALLTHROUGH;
+      [[fallthrough]];
 #endif
 
     case nsIDOMKeyEvent::DOM_VK_LEFT:
@@ -2380,7 +2380,7 @@ nsXULPopupManager::HandleKeyboardEventWithKeyCode(
         break;
       }
       // Intentional fall-through to RETURN case
-      MOZ_FALLTHROUGH;
+      [[fallthrough]];
 
     case nsIDOMKeyEvent::DOM_VK_RETURN: {
       // If there is a popup open, check if the current item needs to be opened.
diff --git a/ldap/xpcom/src/nsLDAPConnection.cpp b/ldap/xpcom/src/nsLDAPConnection.cpp
index 834e803b4c..6457af406a 100644
--- a/ldap/xpcom/src/nsLDAPConnection.cpp
+++ b/ldap/xpcom/src/nsLDAPConnection.cpp
@@ -645,7 +645,7 @@ NS_IMETHODIMP nsLDAPConnectionRunnable::Run()
     case LDAP_RES_SEARCH_REFERENCE:
       // XXX what should we do with LDAP_RES_SEARCH_EXTENDED
       operationFinished = false;
-      MOZ_FALLTHROUGH;
+      [[fallthrough]];
     default:
     {
       msg = new nsLDAPMessage;
diff --git a/mailnews/base/search/src/nsMsgFilterService.cpp b/mailnews/base/search/src/nsMsgFilterService.cpp
index c8f52de5aa..9fd0c23e55 100644
--- a/mailnews/base/search/src/nsMsgFilterService.cpp
+++ b/mailnews/base/search/src/nsMsgFilterService.cpp
@@ -609,7 +609,7 @@ nsresult nsMsgFilterAfterTheFact::ApplyFilter()
         //   would not have run if move succeeded.
         m_nextAction = numActions;
         // Fall through to the copy case.
-        MOZ_FALLTHROUGH;
+        [[fallthrough]];
       case nsMsgFilterAction::CopyToFolder:
       {
         nsCString uri;
diff --git a/mailnews/base/search/src/nsMsgSearchAdapter.cpp b/mailnews/base/search/src/nsMsgSearchAdapter.cpp
index a6f8778305..52a76a9dd1 100644
--- a/mailnews/base/search/src/nsMsgSearchAdapter.cpp
+++ b/mailnews/base/search/src/nsMsgSearchAdapter.cpp
@@ -334,7 +334,7 @@ nsresult nsMsgSearchAdapter::EncodeImapTerm (nsIMsgSearchTerm *term, bool really
   case nsMsgSearchAttrib::ToOrCC:
     orHeaderMnemonic = m_kImapCC;
     // fall through to case nsMsgSearchAttrib::To:
-    MOZ_FALLTHROUGH;
+    [[fallthrough]];
   case nsMsgSearchAttrib::To:
     whichMnemonic = m_kImapTo;
     break;
diff --git a/mailnews/base/src/nsMsgDBView.cpp b/mailnews/base/src/nsMsgDBView.cpp
index 5b82390c00..281d715291 100644
--- a/mailnews/base/src/nsMsgDBView.cpp
+++ b/mailnews/base/src/nsMsgDBView.cpp
@@ -6450,7 +6450,7 @@ NS_IMETHODIMP nsMsgDBView::NoteChange(nsMsgViewIndex firstLineChanged,
       // RowCountChanged() will call our GetRowCount()
       mTree->RowCountChanged(firstLineChanged, numChanged);
       mRemovingRow = false;
-      MOZ_FALLTHROUGH;
+      [[fallthrough]];
     case nsMsgViewNotificationCode::all:
       ClearHdrCache();
       break;
@@ -6730,7 +6730,7 @@ nsresult nsMsgDBView::NavigateFromPos(nsMsgNavigationTypeValue motion, nsMsgView
             break;
         case nsMsgNavigationType::firstUnreadMessage:
             startIndex = nsMsgViewIndex_None;        // note fall thru - is this motion ever used?
-            MOZ_FALLTHROUGH;
+            [[fallthrough]];
         case nsMsgNavigationType::nextUnreadMessage:
             for (curIndex = (startIndex == nsMsgViewIndex_None) ? 0 : startIndex; curIndex <= lastIndex && lastIndex != nsMsgViewIndex_None; curIndex++) {
                 uint32_t flags = m_flags[curIndex];
diff --git a/mailnews/base/src/nsMsgGroupView.cpp b/mailnews/base/src/nsMsgGroupView.cpp
index f3d9a27041..1f20a0378b 100644
--- a/mailnews/base/src/nsMsgGroupView.cpp
+++ b/mailnews/base/src/nsMsgGroupView.cpp
@@ -227,7 +227,7 @@ nsresult nsMsgGroupView::HashHdr(nsIMsgDBHdr *msgHdr, nsString& aHashKey)
       break;
     case nsMsgViewSortType::byReceived:
       rcvDate = true;
-      MOZ_FALLTHROUGH;
+      [[fallthrough]];
     case nsMsgViewSortType::byDate:
     {
       uint32_t ageBucket;
@@ -799,7 +799,7 @@ NS_IMETHODIMP nsMsgGroupView::CellTextForColumn(int32_t aRow,
       {
         case nsMsgViewSortType::byReceived:
           rcvDate = true;
-          MOZ_FALLTHROUGH;
+          [[fallthrough]];
         case nsMsgViewSortType::byDate:
         {
           uint32_t ageBucket = 0;
diff --git a/mailnews/compose/src/nsMsgSend.cpp b/mailnews/compose/src/nsMsgSend.cpp
index 919d9bfc54..17a54286ea 100644
--- a/mailnews/compose/src/nsMsgSend.cpp
+++ b/mailnews/compose/src/nsMsgSend.cpp
@@ -174,7 +174,7 @@ static nsresult StripOutGroupNames(char * addresses)
           group = false;
           //end of the group, act like a recipient separator now...
         /* NO BREAK */
-        MOZ_FALLTHROUGH;
+        [[fallthrough]];
       case ',':
         if (!quoted)
         {
diff --git a/mailnews/compose/src/nsSmtpProtocol.cpp b/mailnews/compose/src/nsSmtpProtocol.cpp
index 89607224ed..a2c86a6eba 100644
--- a/mailnews/compose/src/nsSmtpProtocol.cpp
+++ b/mailnews/compose/src/nsSmtpProtocol.cpp
@@ -920,7 +920,7 @@ void nsSmtpProtocol::InitPrefAuthMethods(int32_t authMethodPrefValue)
       MOZ_LOG(SMTPLogModule, mozilla::LogLevel::Error,
           ("SMTP: bad pref authMethod = %d\n", authMethodPrefValue));
       // fall to any
-      MOZ_FALLTHROUGH;
+      [[fallthrough]];
     case nsMsgAuthMethod::anything:
       m_prefAuthMethods =
           SMTP_AUTH_LOGIN_ENABLED | SMTP_AUTH_PLAIN_ENABLED |
diff --git a/mailnews/db/msgdb/src/nsMsgHdr.cpp b/mailnews/db/msgdb/src/nsMsgHdr.cpp
index 7c5cf57097..2ff1cce192 100644
--- a/mailnews/db/msgdb/src/nsMsgHdr.cpp
+++ b/mailnews/db/msgdb/src/nsMsgHdr.cpp
@@ -769,7 +769,7 @@ const char *nsMsgHdr::GetNextReference(const char *startNextRef,
         // intentional fallthrough so whitespaceEndedAt will definitely have
         //  a non-NULL value, just in case the message-id is not valid (no '>')
         //  and the old-school support is desired.
-        MOZ_FALLTHROUGH;
+        [[fallthrough]];
       default:
         if (!whitespaceEndedAt)
             whitespaceEndedAt = ptr;
diff --git a/mailnews/extensions/bayesian-spam-filter/src/nsBayesianFilter.cpp b/mailnews/extensions/bayesian-spam-filter/src/nsBayesianFilter.cpp
index f879c7aa34..b3e732a443 100644
--- a/mailnews/extensions/bayesian-spam-filter/src/nsBayesianFilter.cpp
+++ b/mailnews/extensions/bayesian-spam-filter/src/nsBayesianFilter.cpp
@@ -547,7 +547,7 @@ void Tokenizer::tokenizeHeaders(nsIUTF8StringEnumerator * aHeaderNames, nsIUTF8S
         if (Substring(headerName, 0, 9).Equals("x-mozilla"))
           break;
         // fall through
-        MOZ_FALLTHROUGH;
+        [[fallthrough]];
     case 'u':
         addTokenForHeader(headerName.get(), headerValue);
         break;
diff --git a/mailnews/imap/src/nsImapMailFolder.cpp b/mailnews/imap/src/nsImapMailFolder.cpp
index 1d08472bb9..5b38433923 100644
--- a/mailnews/imap/src/nsImapMailFolder.cpp
+++ b/mailnews/imap/src/nsImapMailFolder.cpp
@@ -3508,7 +3508,7 @@ NS_IMETHODIMP nsImapMailFolder::ApplyFilterHit(nsIMsgFilter *filter, nsIMsgWindo
           msgIsNew = false;
         }
         // note that delete falls through to move.
-        MOZ_FALLTHROUGH;
+        [[fallthrough]];
         case nsMsgFilterAction::MoveToFolder:
         {
           // if moving to a different file, do it.
diff --git a/mailnews/imap/src/nsImapProtocol.cpp b/mailnews/imap/src/nsImapProtocol.cpp
index 97e61a40ef..3839fade69 100644
--- a/mailnews/imap/src/nsImapProtocol.cpp
+++ b/mailnews/imap/src/nsImapProtocol.cpp
@@ -2775,7 +2775,7 @@ void nsImapProtocol::ProcessSelectedStateURL()
       case nsIImapUrl::nsImapExpungeFolder:
         Expunge();
         // note fall through to next cases.
-        MOZ_FALLTHROUGH;
+        [[fallthrough]];
       case nsIImapUrl::nsImapSelectFolder:
       case nsIImapUrl::nsImapSelectNoopFolder:
         if (!moreHeadersToDownload)
@@ -5624,7 +5624,7 @@ void nsImapProtocol::InitPrefAuthMethods(int32_t authMethodPrefValue)
         MOZ_LOG(IMAP, LogLevel::Error,
             ("IMAP: bad pref authMethod = %d\n", authMethodPrefValue));
         // fall to any
-        MOZ_FALLTHROUGH;
+        [[fallthrough]];
       case nsMsgAuthMethod::anything:
         m_prefAuthMethods = kHasAuthOldLoginCapability |
             kHasAuthLoginCapability | kHasAuthPlainCapability |
diff --git a/mailnews/local/src/nsParseMailbox.cpp b/mailnews/local/src/nsParseMailbox.cpp
index da51c03227..3c5dfa02aa 100644
--- a/mailnews/local/src/nsParseMailbox.cpp
+++ b/mailnews/local/src/nsParseMailbox.cpp
@@ -2026,7 +2026,7 @@ NS_IMETHODIMP nsParseNewMailState::ApplyFilterHit(nsIMsgFilter *filter, nsIMsgWi
         }
 
         // FALLTHROUGH
-        MOZ_FALLTHROUGH;
+        [[fallthrough]];
       case nsMsgFilterAction::MoveToFolder:
         // if moving to a different file, do it.
         if (actionTargetFolderUri.get() && !m_inboxUri.Equals(actionTargetFolderUri,
diff --git a/mailnews/local/src/nsPop3Protocol.cpp b/mailnews/local/src/nsPop3Protocol.cpp
index de129a494e..c89867a9d6 100644
--- a/mailnews/local/src/nsPop3Protocol.cpp
+++ b/mailnews/local/src/nsPop3Protocol.cpp
@@ -1639,7 +1639,7 @@ void nsPop3Protocol::InitPrefAuthMethods(int32_t authMethodPrefValue)
       MOZ_LOG(POP3LOGMODULE, LogLevel::Error,
               (POP3LOG("POP: bad pref authMethod = %d\n"), authMethodPrefValue));
       // fall to any
-      MOZ_FALLTHROUGH;
+      [[fallthrough]];
     case nsMsgAuthMethod::anything:
       m_prefAuthMethods = POP3_HAS_AUTH_USER |
           POP3_HAS_AUTH_LOGIN | POP3_HAS_AUTH_PLAIN |
@@ -1802,7 +1802,7 @@ int32_t nsPop3Protocol::ProcessAuth()
         break;
       case POP3_HAS_AUTH_CRAM_MD5:
         MOZ_LOG(POP3LOGMODULE, LogLevel::Debug, (POP3LOG("POP CRAM")));
-        MOZ_FALLTHROUGH;
+        [[fallthrough]];
       case POP3_HAS_AUTH_PLAIN:
       case POP3_HAS_AUTH_USER:
         MOZ_LOG(POP3LOGMODULE, LogLevel::Debug, (POP3LOG("POP username")));
diff --git a/mailnews/mime/src/mimemsig.cpp b/mailnews/mime/src/mimemsig.cpp
index 5b4f8e556a..1ad78f3ed9 100644
--- a/mailnews/mime/src/mimemsig.cpp
+++ b/mailnews/mime/src/mimemsig.cpp
@@ -256,7 +256,7 @@ MimeMultipartSigned_parse_line (const char *line, int32_t length, MimeObject *ob
     mult->hdrs = 0;
 
     /* fall through. */
-   MOZ_FALLTHROUGH;
+   [[fallthrough]];
   case MimeMultipartSignedBodyFirstHeader:
   case MimeMultipartSignedBodyHeaders:
   case MimeMultipartSignedBodyLine:
@@ -406,7 +406,7 @@ MimeMultipartSigned_parse_line (const char *line, int32_t length, MimeObject *ob
     }
 
     /* fall through. */
-   MOZ_FALLTHROUGH;
+   [[fallthrough]];
   case MimeMultipartSignedSignatureLine:
     if (hash_line_p)
     {
@@ -497,7 +497,7 @@ MimeMultipartSigned_parse_child_line (MimeObject *obj,
   case MimeMultipartSignedBodyHeaders:
     // How'd we get here?  Oh well, fall through.
     NS_ERROR("wrong state in parse child line");
-    MOZ_FALLTHROUGH;
+    [[fallthrough]];
   case MimeMultipartSignedBodyFirstLine:
     PR_ASSERT(first_line_p);
     if (!sig->part_buffer)
@@ -507,7 +507,7 @@ MimeMultipartSigned_parse_child_line (MimeObject *obj,
       return MIME_OUT_OF_MEMORY;
     }
     /* fall through */
-   MOZ_FALLTHROUGH;
+   [[fallthrough]];
   case MimeMultipartSignedBodyLine:
     {
     /* This is the first part; we are buffering it, and will emit it all
@@ -551,7 +551,7 @@ MimeMultipartSigned_parse_child_line (MimeObject *obj,
   case MimeMultipartSignedSignatureHeaders:
     // How'd we get here?  Oh well, fall through.
     NS_ERROR("should have already parse sig hdrs");
-    MOZ_FALLTHROUGH;
+    [[fallthrough]];
   case MimeMultipartSignedSignatureFirstLine:
   case MimeMultipartSignedSignatureLine:
     /* Nothing to do here -- hashing of the signature part is handled up
diff --git a/mailnews/news/src/nsNNTPProtocol.cpp b/mailnews/news/src/nsNNTPProtocol.cpp
index 035dff6e6f..c5a426947d 100644
--- a/mailnews/news/src/nsNNTPProtocol.cpp
+++ b/mailnews/news/src/nsNNTPProtocol.cpp
@@ -4549,12 +4549,12 @@ nsresult nsNNTPProtocol::ProcessProtocolState(nsIURI * url, nsIInputStream * inp
       FinishMemCacheEntry(false);  // cleanup mem cache entry
       if (m_responseCode != MK_NNTP_RESPONSE_ARTICLE_NOTFOUND && m_responseCode != MK_NNTP_RESPONSE_ARTICLE_NONEXIST)
         return CloseConnection();
-      MOZ_FALLTHROUGH;
+      [[fallthrough]];
     case NEWS_FREE:
       // Remember when we last used this connection
       m_lastActiveTimeStamp = PR_Now();
       CleanupAfterRunningUrl();
-      MOZ_FALLTHROUGH;
+      [[fallthrough]];
     case NNTP_SUSPENDED:
       return NS_OK;
       break;
diff --git a/media/mtransport/transportlayerdtls.cpp b/media/mtransport/transportlayerdtls.cpp
index 8522919eef..a13b8c4334 100644
--- a/media/mtransport/transportlayerdtls.cpp
+++ b/media/mtransport/transportlayerdtls.cpp
@@ -895,7 +895,7 @@ void TransportLayerDtls::Handshake() {
         MOZ_MTLOG(ML_ERROR, LAYER_INFO << "Malformed DTLS message; ignoring");
         // If this were TLS (and not DTLS), this would be fatal, but
         // here we're required to ignore bad messages, so fall through
-        MOZ_FALLTHROUGH;
+        [[fallthrough]];
       case PR_WOULD_BLOCK_ERROR:
         MOZ_MTLOG(ML_NOTICE, LAYER_INFO << "Handshake would have blocked");
         PRIntervalTime timeout;
diff --git a/mfbt/Assertions.h b/mfbt/Assertions.h
index 09a1b8eaa6..a965ae6f1d 100644
--- a/mfbt/Assertions.h
+++ b/mfbt/Assertions.h
@@ -554,7 +554,7 @@ struct AssertionConditionType
  *   default:
  *     // This case wants to assert in debug builds, fall through in release.
  *     MOZ_ASSERT(false); // -Wimplicit-fallthrough warning in release builds!
- *     MOZ_FALLTHROUGH;   // but -Wunreachable-code warning in debug builds!
+ *     [[fallthrough]];   // but -Wunreachable-code warning in debug builds!
  *   case 5:
  *     return 5;
  * }
@@ -571,7 +571,7 @@ struct AssertionConditionType
 #ifdef DEBUG
 #  define MOZ_FALLTHROUGH_ASSERT(reason) MOZ_CRASH("MOZ_FALLTHROUGH_ASSERT: " reason)
 #else
-#  define MOZ_FALLTHROUGH_ASSERT(...) MOZ_FALLTHROUGH
+#  define MOZ_FALLTHROUGH_ASSERT(...) [[fallthrough]]
 #endif
 
 /*
diff --git a/mfbt/Attributes.h b/mfbt/Attributes.h
index fdd253d357..5da4ffe948 100644
--- a/mfbt/Attributes.h
+++ b/mfbt/Attributes.h
@@ -238,50 +238,6 @@
 
 #ifdef __cplusplus
 
-/**
- * MOZ_FALLTHROUGH is an annotation to suppress compiler warnings about switch
- * cases that fall through without a break or return statement. MOZ_FALLTHROUGH
- * is only needed on cases that have code.
- *
- * MOZ_FALLTHROUGH_ASSERT is an annotation to suppress compiler warnings about
- * switch cases that MOZ_ASSERT(false) (or its alias MOZ_ASSERT_UNREACHABLE) in
- * debug builds, but intentionally fall through in release builds. See comment
- * in Assertions.h for more details.
- *
- * switch (foo) {
- *   case 1: // These cases have no code. No fallthrough annotations are needed.
- *   case 2:
- *   case 3: // This case has code, so a fallthrough annotation is needed!
- *     foo++;
- *     MOZ_FALLTHROUGH;
- *   case 4:
- *     return foo;
- *
- *   default:
- *     // This case asserts in debug builds, falls through in release.
- *     MOZ_FALLTHROUGH_ASSERT("Unexpected foo value?!");
- *   case 5:
- *     return 5;
- * }
- */
-#ifndef __has_cpp_attribute
-#  define __has_cpp_attribute(x) 0
-#endif
-
-#if __has_cpp_attribute(clang::fallthrough)
-#  define MOZ_FALLTHROUGH [[clang::fallthrough]]
-#elif __has_cpp_attribute(gnu::fallthrough)
-#  define MOZ_FALLTHROUGH [[gnu::fallthrough]]
-#elif defined(_MSC_VER)
-   /*
-    * MSVC's __fallthrough annotations are checked by /analyze (Code Analysis):
-    * https://msdn.microsoft.com/en-us/library/ms235402%28VS.80%29.aspx
-    */
-#  include <sal.h>
-#  define MOZ_FALLTHROUGH __fallthrough
-#else
-#  define MOZ_FALLTHROUGH /* FALLTHROUGH */
-#endif
 /*
  * MOZ_ASAN_BLACKLIST is a macro to tell AddressSanitizer (a compile-time
  * instrumentation shipped with Clang and GCC) to not instrument the annotated
diff --git a/modules/libjar/nsJARInputStream.cpp b/modules/libjar/nsJARInputStream.cpp
index a1f6c1d560..a26cd38ea8 100644
--- a/modules/libjar/nsJARInputStream.cpp
+++ b/modules/libjar/nsJARInputStream.cpp
@@ -118,7 +118,7 @@ nsJARInputStream::InitDirectory(nsJAR* aJar,
             case ')':
             case '\\':
                 escDirName.Append('\\');
-                MOZ_FALLTHROUGH;
+                [[fallthrough]];
             default:
                 escDirName.Append(*curr);
         }
diff --git a/mozglue/linker/XZStream.cpp b/mozglue/linker/XZStream.cpp
index bd71655f52..7ddbd875ac 100644
--- a/mozglue/linker/XZStream.cpp
+++ b/mozglue/linker/XZStream.cpp
@@ -94,11 +94,11 @@ XZStream::Decode(void* aOutBuf, size_t aOutSize)
       case XZ_STREAM_END:
         // Stream ended, the next loop iteration should terminate.
         MOZ_ASSERT(mBuffers.in_pos == mBuffers.in_size);
-        MOZ_FALLTHROUGH;
+        [[fallthrough]];
 #ifdef XZ_DEC_ANY_CHECK
       case XZ_UNSUPPORTED_CHECK:
         // Ignore unsupported check.
-        MOZ_FALLTHROUGH;
+        [[fallthrough]];
 #endif
       case XZ_OK:
         // Chunk decoded, proceed.
@@ -121,7 +121,7 @@ XZStream::Decode(void* aOutBuf, size_t aOutSize)
         return 0;
 
       case XZ_DATA_ERROR:
-        MOZ_FALLTHROUGH;
+        [[fallthrough]];
       case XZ_BUF_ERROR:
         ERROR("XZ decoding: corrupt input stream");
         return 0;
diff --git a/netwerk/base/nsURLHelper.cpp b/netwerk/base/nsURLHelper.cpp
index 0ddbdb6a6a..1024b2ef53 100644
--- a/netwerk/base/nsURLHelper.cpp
+++ b/netwerk/base/nsURLHelper.cpp
@@ -427,7 +427,7 @@ net_ResolveRelativePath(const nsACString &relativePath,
           case '#':
           case '?':
             stop = true;
-            MOZ_FALLTHROUGH;
+            [[fallthrough]];
           case '/':
             // delimiter found
             if (name.EqualsLiteral("..")) {
diff --git a/netwerk/cache/nsDiskCacheDevice.cpp b/netwerk/cache/nsDiskCacheDevice.cpp
index 05bd6e06f2..c18ad012af 100644
--- a/netwerk/cache/nsDiskCacheDevice.cpp
+++ b/netwerk/cache/nsDiskCacheDevice.cpp
@@ -303,17 +303,17 @@ nsDiskCache::Hash(const char * key, PLDHashNumber initval)
   /*------------------------------------- handle the last 11 bytes */
   c += length;
   switch(len) {              /* all the case statements fall through */
-    case 11: c += (uint32_t(k[10])<<24);  MOZ_FALLTHROUGH;
-    case 10: c += (uint32_t(k[9])<<16);   MOZ_FALLTHROUGH;
-    case 9 : c += (uint32_t(k[8])<<8);    MOZ_FALLTHROUGH;
+    case 11: c += (uint32_t(k[10])<<24);  [[fallthrough]];
+    case 10: c += (uint32_t(k[9])<<16);   [[fallthrough]];
+    case 9 : c += (uint32_t(k[8])<<8);    [[fallthrough]];
     /* the low-order byte of c is reserved for the length */
-    case 8 : b += (uint32_t(k[7])<<24);   MOZ_FALLTHROUGH;
-    case 7 : b += (uint32_t(k[6])<<16);   MOZ_FALLTHROUGH;
-    case 6 : b += (uint32_t(k[5])<<8);    MOZ_FALLTHROUGH;
-    case 5 : b += k[4];                   MOZ_FALLTHROUGH;
-    case 4 : a += (uint32_t(k[3])<<24);   MOZ_FALLTHROUGH;
-    case 3 : a += (uint32_t(k[2])<<16);   MOZ_FALLTHROUGH;
-    case 2 : a += (uint32_t(k[1])<<8);    MOZ_FALLTHROUGH;
+    case 8 : b += (uint32_t(k[7])<<24);   [[fallthrough]];
+    case 7 : b += (uint32_t(k[6])<<16);   [[fallthrough]];
+    case 6 : b += (uint32_t(k[5])<<8);    [[fallthrough]];
+    case 5 : b += k[4];                   [[fallthrough]];
+    case 4 : a += (uint32_t(k[3])<<24);   [[fallthrough]];
+    case 3 : a += (uint32_t(k[2])<<16);   [[fallthrough]];
+    case 2 : a += (uint32_t(k[1])<<8);    [[fallthrough]];
     case 1 : a += k[0];
     /* case 0: nothing left to add */
   }
diff --git a/netwerk/cache2/CacheHashUtils.cpp b/netwerk/cache2/CacheHashUtils.cpp
index 6783f0b869..3dd169c420 100644
--- a/netwerk/cache2/CacheHashUtils.cpp
+++ b/netwerk/cache2/CacheHashUtils.cpp
@@ -58,17 +58,17 @@ CacheHash::Hash(const char *aData, uint32_t aSize, uint32_t aInitval)
   /*------------------------------------- handle the last 11 bytes */
   c += aSize;
   switch(len) {              /* all the case statements fall through */
-    case 11: c += (uint32_t(k[10])<<24);  MOZ_FALLTHROUGH;
-    case 10: c += (uint32_t(k[9])<<16);   MOZ_FALLTHROUGH;
-    case 9 : c += (uint32_t(k[8])<<8);    MOZ_FALLTHROUGH;
+    case 11: c += (uint32_t(k[10])<<24);  [[fallthrough]];
+    case 10: c += (uint32_t(k[9])<<16);   [[fallthrough]];
+    case 9 : c += (uint32_t(k[8])<<8);    [[fallthrough]];
     /* the low-order byte of c is reserved for the length */
-    case 8 : b += (uint32_t(k[7])<<24);   MOZ_FALLTHROUGH;
-    case 7 : b += (uint32_t(k[6])<<16);   MOZ_FALLTHROUGH;
-    case 6 : b += (uint32_t(k[5])<<8);    MOZ_FALLTHROUGH;
-    case 5 : b += k[4];                   MOZ_FALLTHROUGH;
-    case 4 : a += (uint32_t(k[3])<<24);   MOZ_FALLTHROUGH;
-    case 3 : a += (uint32_t(k[2])<<16);   MOZ_FALLTHROUGH;
-    case 2 : a += (uint32_t(k[1])<<8);    MOZ_FALLTHROUGH;
+    case 8 : b += (uint32_t(k[7])<<24);   [[fallthrough]];
+    case 7 : b += (uint32_t(k[6])<<16);   [[fallthrough]];
+    case 6 : b += (uint32_t(k[5])<<8);    [[fallthrough]];
+    case 5 : b += k[4];                   [[fallthrough]];
+    case 4 : a += (uint32_t(k[3])<<24);   [[fallthrough]];
+    case 3 : a += (uint32_t(k[2])<<16);   [[fallthrough]];
+    case 2 : a += (uint32_t(k[1])<<8);    [[fallthrough]];
     case 1 : a += k[0];
     /* case 0: nothing left to add */
   }
@@ -158,8 +158,8 @@ CacheHash::Update(const char *aData, uint32_t aLen)
   }
 
   switch (aLen) {
-    case 3: mBuf += data[2] << 16;  MOZ_FALLTHROUGH;
-    case 2: mBuf += data[1] << 8;   MOZ_FALLTHROUGH;
+    case 3: mBuf += data[2] << 16;  [[fallthrough]];
+    case 2: mBuf += data[1] << 8;   [[fallthrough]];
     case 1: mBuf += data[0];
   }
 
diff --git a/netwerk/cache2/CacheIndex.cpp b/netwerk/cache2/CacheIndex.cpp
index 0c46647c63..1fae397278 100644
--- a/netwerk/cache2/CacheIndex.cpp
+++ b/netwerk/cache2/CacheIndex.cpp
@@ -152,7 +152,7 @@ private:
         if (!mDoNotSearchInUpdates) {
           entry = mIndex->mPendingUpdates.GetEntry(*mHash);
         }
-        MOZ_FALLTHROUGH;
+        [[fallthrough]];
       case CacheIndex::BUILDING:
       case CacheIndex::UPDATING:
       case CacheIndex::READY:
@@ -459,7 +459,7 @@ CacheIndex::Shutdown()
   switch (oldState) {
     case WRITING:
       index->FinishWrite(false, lock);
-      MOZ_FALLTHROUGH;
+      [[fallthrough]];
     case READY:
       if (index->mIndexOnDiskIsValid && !index->mDontMarkIndexClean) {
         if (!sanitize && NS_FAILED(index->WriteLogToDisk())) {
@@ -1153,7 +1153,7 @@ CacheIndex::HasEntry(const SHA1Sum::Hash &hash, EntryStatus *_retval, bool *_pin
     case READING:
     case WRITING:
       entry = index->mPendingUpdates.GetEntry(hash);
-      MOZ_FALLTHROUGH;
+      [[fallthrough]];
     case BUILDING:
     case UPDATING:
     case READY:
diff --git a/netwerk/cache2/CacheStorageService.cpp b/netwerk/cache2/CacheStorageService.cpp
index 0ed854e916..9890ce4806 100644
--- a/netwerk/cache2/CacheStorageService.cpp
+++ b/netwerk/cache2/CacheStorageService.cpp
@@ -450,7 +450,7 @@ private:
         }
 
         mPass = ITERATE_METADATA;
-        MOZ_FALLTHROUGH;
+        [[fallthrough]];
 
       case ITERATE_METADATA:
         // Now grab the context iterator.
diff --git a/netwerk/cookie/nsCookieService.cpp b/netwerk/cookie/nsCookieService.cpp
index 2549ffaccb..d08f7fc3c5 100644
--- a/netwerk/cookie/nsCookieService.cpp
+++ b/netwerk/cookie/nsCookieService.cpp
@@ -992,7 +992,7 @@ nsCookieService::TryInitDB(bool aRecreateDB)
         NS_ENSURE_SUCCESS(rv, RESULT_RETRY);
       }
       // Fall through to the next upgrade.
-      MOZ_FALLTHROUGH;
+      [[fallthrough]];
 
     case 2:
       {
@@ -1051,7 +1051,7 @@ nsCookieService::TryInitDB(bool aRecreateDB)
         NS_ENSURE_SUCCESS(rv, RESULT_RETRY);
       }
       // Fall through to the next upgrade.
-      MOZ_FALLTHROUGH;
+      [[fallthrough]];
 
     case 3:
       {
@@ -1148,7 +1148,7 @@ nsCookieService::TryInitDB(bool aRecreateDB)
         NS_ENSURE_SUCCESS(rv, RESULT_RETRY);
       }
       // Fall through to the next upgrade.
-      MOZ_FALLTHROUGH;
+      [[fallthrough]];
 
     case 4:
       {
@@ -1196,7 +1196,7 @@ nsCookieService::TryInitDB(bool aRecreateDB)
           ("Upgraded database to schema version 5"));
       }
       // Fall through to the next upgrade.
-      MOZ_FALLTHROUGH;
+      [[fallthrough]];
 
     case 5:
       {
@@ -1262,7 +1262,7 @@ nsCookieService::TryInitDB(bool aRecreateDB)
         COOKIE_LOGSTRING(LogLevel::Debug,
           ("Upgraded database to schema version 6"));
       }
-      MOZ_FALLTHROUGH;
+      [[fallthrough]];
 
     case 6:
       {
@@ -1322,7 +1322,7 @@ nsCookieService::TryInitDB(bool aRecreateDB)
       // No more upgrades. Update the schema version.
       rv = mDefaultDBState->dbConn->SetSchemaVersion(COOKIES_SCHEMA_VERSION);
       NS_ENSURE_SUCCESS(rv, RESULT_RETRY);
-      MOZ_FALLTHROUGH;
+      [[fallthrough]];
 
     case COOKIES_SCHEMA_VERSION:
       break;
@@ -1340,7 +1340,7 @@ nsCookieService::TryInitDB(bool aRecreateDB)
         NS_ENSURE_SUCCESS(rv, RESULT_RETRY);
       }
       // fall through to downgrade check
-      MOZ_FALLTHROUGH;
+      [[fallthrough]];
 
     // downgrading.
     // if columns have been added to the table, we can still use the ones we
diff --git a/netwerk/protocol/http/Http2Stream.cpp b/netwerk/protocol/http/Http2Stream.cpp
index 9b8f13c2d2..bcb7f73362 100644
--- a/netwerk/protocol/http/Http2Stream.cpp
+++ b/netwerk/protocol/http/Http2Stream.cpp
@@ -1357,7 +1357,7 @@ Http2Stream::OnReadSegment(const char *buf,
     mRequestBodyLenRemaining -= dataLength;
     GenerateDataFrameHeader(dataLength, !mRequestBodyLenRemaining);
     ChangeState(SENDING_BODY);
-    MOZ_FALLTHROUGH;
+    [[fallthrough]];
 
   case SENDING_BODY:
     MOZ_ASSERT(mTxInlineFrameUsed, "OnReadSegment Send Data Header 0b");
diff --git a/netwerk/protocol/http/HttpBaseChannel.cpp b/netwerk/protocol/http/HttpBaseChannel.cpp
index 0b4929bbc8..fe8bbc5efa 100644
--- a/netwerk/protocol/http/HttpBaseChannel.cpp
+++ b/netwerk/protocol/http/HttpBaseChannel.cpp
@@ -1577,7 +1577,7 @@ HttpBaseChannel::SetReferrerWithPolicy(nsIURI *referrer,
         // No URL, so fall through to truncating the path and any query/ref off
         // as well.
       }
-      MOZ_FALLTHROUGH;
+      [[fallthrough]];
       default: // (Pref limited to [0,2] enforced by clamp, MOZ_CRASH overkill.)
       case 2: // scheme+host+port+/
         spec.AppendLiteral("/");
diff --git a/netwerk/protocol/http/nsHttpTransaction.cpp b/netwerk/protocol/http/nsHttpTransaction.cpp
index 6c6865c239..6532adf24a 100644
--- a/netwerk/protocol/http/nsHttpTransaction.cpp
+++ b/netwerk/protocol/http/nsHttpTransaction.cpp
@@ -1668,7 +1668,7 @@ nsHttpTransaction::HandleContentStart()
         switch (mResponseHead->Status()) {
         case 101:
             mPreserveStream = true;
-            MOZ_FALLTHROUGH; // to other no content cases:
+            [[fallthrough]]; // to other no content cases:
         case 204:
         case 205:
         case 304:
diff --git a/netwerk/protocol/websocket/WebSocketChannel.cpp b/netwerk/protocol/websocket/WebSocketChannel.cpp
index 9fca09a087..d645a96afd 100644
--- a/netwerk/protocol/websocket/WebSocketChannel.cpp
+++ b/netwerk/protocol/websocket/WebSocketChannel.cpp
@@ -2153,7 +2153,7 @@ WebSocketChannel::PrimeNewOutgoingMessage()
       msgType = kMsgTypeBinaryString;
 
       // no break: fall down into binary string case
-      MOZ_FALLTHROUGH;
+      [[fallthrough]];
 
     case kMsgTypeBinaryString:
       mOutHeader[0] = kFinalFragBit | nsIWebSocketFrame::OPCODE_BINARY;
diff --git a/netwerk/sctp/datachannel/DataChannel.h b/netwerk/sctp/datachannel/DataChannel.h
index 02dca930c5..db0e2f7f75 100644
--- a/netwerk/sctp/datachannel/DataChannel.h
+++ b/netwerk/sctp/datachannel/DataChannel.h
@@ -544,7 +544,7 @@ public:
       case ON_DISCONNECTED:
         // If we've disconnected, make sure we close all the streams - from mainthread!
         mConnection->CloseAll();
-        MOZ_FALLTHROUGH;
+        [[fallthrough]];
       case ON_CHANNEL_CREATED:
       case ON_CONNECTION:
         // WeakPtr - only used/modified/nulled from MainThread so we can use a WeakPtr here
diff --git a/netwerk/streamconv/converters/mozTXTToHTMLConv.cpp b/netwerk/streamconv/converters/mozTXTToHTMLConv.cpp
index db23cca35f..c5be10b216 100644
--- a/netwerk/streamconv/converters/mozTXTToHTMLConv.cpp
+++ b/netwerk/streamconv/converters/mozTXTToHTMLConv.cpp
@@ -53,7 +53,7 @@ mozTXTToHTMLConv::EscapeChar(const char16_t ch, nsString& aStringToAppendTo,
         break;
       }
       // else fall through
-      MOZ_FALLTHROUGH;
+      [[fallthrough]];
     default:
       aStringToAppendTo += ch;
     }
@@ -101,7 +101,7 @@ mozTXTToHTMLConv::EscapeStr(nsString& aInString, bool inAttribute)
         break;
       }
       // else fall through
-      MOZ_FALLTHROUGH;
+      [[fallthrough]];
     default:
       i++;
     }
@@ -512,7 +512,7 @@ mozTXTToHTMLConv::FindURL(const char16_t * aInString, int32_t aInLength, const u
   {
   case '@':
     state[RFC2396E] = unchecked;
-    MOZ_FALLTHROUGH;
+    [[fallthrough]];
   case '.':
     state[abbreviated] = unchecked;
     break;
diff --git a/netwerk/streamconv/converters/nsHTTPCompressConv.cpp b/netwerk/streamconv/converters/nsHTTPCompressConv.cpp
index b540240322..6030e1c8d6 100644
--- a/netwerk/streamconv/converters/nsHTTPCompressConv.cpp
+++ b/netwerk/streamconv/converters/nsHTTPCompressConv.cpp
@@ -291,7 +291,7 @@ nsHTTPCompressConv::OnDataAvailable(nsIRequest* request,
         return NS_OK;
       }
 
-      MOZ_FALLTHROUGH;
+      [[fallthrough]];
 
     case HTTP_COMPRESS_DEFLATE:
 
diff --git a/parser/html/nsHtml5Highlighter.cpp b/parser/html/nsHtml5Highlighter.cpp
index b08179fbd2..7f393a5490 100644
--- a/parser/html/nsHtml5Highlighter.cpp
+++ b/parser/html/nsHtml5Highlighter.cpp
@@ -577,7 +577,7 @@ nsHtml5Highlighter::FlushChars()
           // the input data, because there are no reparses in the View Source
           // case, so we won't need the original data in the buffer anymore.
           buf[i] = '\n';
-          MOZ_FALLTHROUGH;
+          [[fallthrough]];
         case '\n': {
           ++i;
           if (mCStart < i) {
diff --git a/security/manager/ssl/nsNSSComponent.cpp b/security/manager/ssl/nsNSSComponent.cpp
index 05527cf9b0..7aad79a127 100644
--- a/security/manager/ssl/nsNSSComponent.cpp
+++ b/security/manager/ssl/nsNSSComponent.cpp
@@ -172,7 +172,7 @@ bool EnsureNSSInitialized(EnsureNSSOperator op)
   case nssInitFailed:
     NS_ASSERTION(loading, "Bad call to EnsureNSSInitialized(nssInitFailed)");
     loading = false;
-    MOZ_FALLTHROUGH;
+    [[fallthrough]];
 
   case nssShutdown:
     PR_AtomicSet(&haveLoaded, 0);
diff --git a/storage/mozStoragePrivateHelpers.cpp b/storage/mozStoragePrivateHelpers.cpp
index 0c4a890504..5aa72b2b65 100644
--- a/storage/mozStoragePrivateHelpers.cpp
+++ b/storage/mozStoragePrivateHelpers.cpp
@@ -229,7 +229,7 @@ convertVariantToStorageVariant(nsIVariant* aVariant)
         // Take ownership of the data avoiding a further copy.
         return new AdoptedBlobVariant(v);
       }
-      MOZ_FALLTHROUGH;
+      [[fallthrough]];
     }
     case nsIDataType::VTYPE_EMPTY:
     case nsIDataType::VTYPE_EMPTY_ARRAY:
diff --git a/toolkit/components/downloads/nsDownloadManager.cpp b/toolkit/components/downloads/nsDownloadManager.cpp
index 587c1ac8ab..d2eccc7b64 100644
--- a/toolkit/components/downloads/nsDownloadManager.cpp
+++ b/toolkit/components/downloads/nsDownloadManager.cpp
@@ -491,7 +491,7 @@ nsDownloadManager::InitFileDB()
       NS_ENSURE_SUCCESS(rv, rv);
     }
     // Fallthrough to the next upgrade
-    MOZ_FALLTHROUGH;
+    [[fallthrough]];
 
   case 2: // Add referrer column to the database
     {
@@ -506,7 +506,7 @@ nsDownloadManager::InitFileDB()
       NS_ENSURE_SUCCESS(rv, rv);
     }
     // Fallthrough to the next upgrade
-    MOZ_FALLTHROUGH;
+    [[fallthrough]];
 
   case 3: // This version adds a column to the database (entityID)
     {
@@ -521,7 +521,7 @@ nsDownloadManager::InitFileDB()
       NS_ENSURE_SUCCESS(rv, rv);
     }
     // Fallthrough to the next upgrade
-    MOZ_FALLTHROUGH;
+    [[fallthrough]];
 
   case 4: // This version adds a column to the database (tempPath)
     {
@@ -536,7 +536,7 @@ nsDownloadManager::InitFileDB()
       NS_ENSURE_SUCCESS(rv, rv);
     }
     // Fallthrough to the next upgrade
-    MOZ_FALLTHROUGH;
+    [[fallthrough]];
 
   case 5: // This version adds two columns for tracking transfer progress
     {
@@ -556,7 +556,7 @@ nsDownloadManager::InitFileDB()
       NS_ENSURE_SUCCESS(rv, rv);
     }
     // Fallthrough to the next upgrade
-    MOZ_FALLTHROUGH;
+    [[fallthrough]];
 
   case 6: // This version adds three columns to DB (MIME type related info)
     {
@@ -581,7 +581,7 @@ nsDownloadManager::InitFileDB()
       NS_ENSURE_SUCCESS(rv, rv);
     }
     // Fallthrough to next upgrade
-    MOZ_FALLTHROUGH;
+    [[fallthrough]];
 
   case 7: // This version adds a column to remember to auto-resume downloads
     {
@@ -596,7 +596,7 @@ nsDownloadManager::InitFileDB()
       NS_ENSURE_SUCCESS(rv, rv);
     }
     // Fallthrough to the next upgrade
-    MOZ_FALLTHROUGH;
+    [[fallthrough]];
 
     // Warning: schema versions >=8 must take into account that they can
     // be operating on schemas from unknown, future versions that have
@@ -631,7 +631,7 @@ nsDownloadManager::InitFileDB()
 
   // Extra sanity checking for developers
 #ifndef DEBUG
-    MOZ_FALLTHROUGH;
+    [[fallthrough]];
   case DM_SCHEMA_VERSION:
 #endif
     break;
@@ -648,7 +648,7 @@ nsDownloadManager::InitFileDB()
       NS_ENSURE_SUCCESS(rv, rv);
     }
     // Fallthrough to downgrade check
-    MOZ_FALLTHROUGH;
+    [[fallthrough]];
 
   // Downgrading
   // If columns have been added to the table, we can still use the ones we
diff --git a/toolkit/components/places/nsAnnotationService.cpp b/toolkit/components/places/nsAnnotationService.cpp
index 9d62bd34a8..9e42836144 100644
--- a/toolkit/components/places/nsAnnotationService.cpp
+++ b/toolkit/components/places/nsAnnotationService.cpp
@@ -211,7 +211,7 @@ nsAnnotationService::SetPageAnnotation(nsIURI* aURI,
         return NS_OK;
       }
       // Fall through int64_t case otherwise.
-      MOZ_FALLTHROUGH;
+      [[fallthrough]];
     }
     case nsIDataType::VTYPE_INT64:
     case nsIDataType::VTYPE_UINT64: {
@@ -224,7 +224,7 @@ nsAnnotationService::SetPageAnnotation(nsIURI* aURI,
         return NS_OK;
       }
       // Fall through double case otherwise.
-      MOZ_FALLTHROUGH;
+      [[fallthrough]];
     }
     case nsIDataType::VTYPE_FLOAT:
     case nsIDataType::VTYPE_DOUBLE: {
@@ -296,7 +296,7 @@ nsAnnotationService::SetItemAnnotation(int64_t aItemId,
         return NS_OK;
       }
       // Fall through int64_t case otherwise.
-      MOZ_FALLTHROUGH;
+      [[fallthrough]];
     }
     case nsIDataType::VTYPE_INT64:
     case nsIDataType::VTYPE_UINT64: {
@@ -309,7 +309,7 @@ nsAnnotationService::SetItemAnnotation(int64_t aItemId,
         return NS_OK;
       }
       // Fall through double case otherwise.
-      MOZ_FALLTHROUGH;
+      [[fallthrough]];
     }
     case nsIDataType::VTYPE_FLOAT:
     case nsIDataType::VTYPE_DOUBLE: {
diff --git a/toolkit/components/places/nsNavHistoryResult.cpp b/toolkit/components/places/nsNavHistoryResult.cpp
index 64d06308fd..f07487dfcd 100644
--- a/toolkit/components/places/nsNavHistoryResult.cpp
+++ b/toolkit/components/places/nsNavHistoryResult.cpp
@@ -2452,7 +2452,7 @@ nsNavHistoryQueryResultNode::OnVisit(nsIURI* aURI, int64_t aVisitId,
 
       // Fall through to check the time, if the time is not present it will
       // still match.
-      MOZ_FALLTHROUGH;
+      [[fallthrough]];
     }
 
     case QUERYUPDATE_TIME: {
@@ -2479,7 +2479,7 @@ nsNavHistoryQueryResultNode::OnVisit(nsIURI* aURI, int64_t aVisitId,
       }
       // Now we know that our visit satisfies the time range, fall through to
       // the QUERYUPDATE_SIMPLE case below.
-      MOZ_FALLTHROUGH;
+      [[fallthrough]];
     }
 
     case QUERYUPDATE_SIMPLE: {
@@ -2917,7 +2917,7 @@ nsNavHistoryQueryResultNode::OnItemChanged(int64_t aItemId,
         // folders titles.
         if (mOptions->ResultType() != nsINavHistoryQueryOptions::RESULTS_AS_TAG_QUERY)
           return NS_OK;
-        MOZ_FALLTHROUGH;
+        [[fallthrough]];
       default:
         (void)Refresh();
     }
diff --git a/toolkit/components/satchel/nsFormFillController.cpp b/toolkit/components/satchel/nsFormFillController.cpp
index aac1870490..69354930e0 100644
--- a/toolkit/components/satchel/nsFormFillController.cpp
+++ b/toolkit/components/satchel/nsFormFillController.cpp
@@ -1062,7 +1062,7 @@ nsFormFillController::KeyPress(nsIDOMEvent* aEvent)
       if (isCtrl || isAlt || isMeta)
         break;
     }
-    MOZ_FALLTHROUGH;
+    [[fallthrough]];
   case nsIDOMKeyEvent::DOM_VK_UP:
   case nsIDOMKeyEvent::DOM_VK_DOWN:
   case nsIDOMKeyEvent::DOM_VK_LEFT:
diff --git a/toolkit/mozapps/update/updater/updater.cpp b/toolkit/mozapps/update/updater/updater.cpp
index f5f71935dc..073c0452fb 100644
--- a/toolkit/mozapps/update/updater/updater.cpp
+++ b/toolkit/mozapps/update/updater/updater.cpp
@@ -3764,7 +3764,7 @@ int add_dir_entries(const NS_tchar *dirpath, ActionList *list)
         LOG(("add_dir_entries: found a non-standard file: " LOG_S,
              ftsdirEntry->fts_path));
         // Fall through and try to remove as a file
-        MOZ_FALLTHROUGH;
+        [[fallthrough]];
 
       // Files
       case FTS_F:
@@ -3813,7 +3813,7 @@ int add_dir_entries(const NS_tchar *dirpath, ActionList *list)
           rv = OK;
           break;
         }
-        MOZ_FALLTHROUGH;
+        [[fallthrough]];
 
       case FTS_ERR:
         rv = UNEXPECTED_FILE_OPERATION_ERROR;
diff --git a/toolkit/xre/nsUpdateDriver.cpp b/toolkit/xre/nsUpdateDriver.cpp
index 577d74e6f5..69b6333b83 100644
--- a/toolkit/xre/nsUpdateDriver.cpp
+++ b/toolkit/xre/nsUpdateDriver.cpp
@@ -961,7 +961,7 @@ ProcessUpdates(nsIFile *greDir, nsIFile *appDir, nsIFile *updRootDir,
       break;
     }
     // Intentional fallthrough to ePendingUpdate.
-    MOZ_FALLTHROUGH;
+    [[fallthrough]];
   }
   case ePendingUpdate: {
     ApplyUpdate(greDir, updatesDir, statusFile, appDir, argc, argv, restart,
diff --git a/tools/power/rapl.cpp b/tools/power/rapl.cpp
index 54c00f79f8..33cc2d781e 100644
--- a/tools/power/rapl.cpp
+++ b/tools/power/rapl.cpp
@@ -345,7 +345,7 @@ public:
 
       case 63:  // 0x3f: Haswell-Server
         mHasRamUnitsQuirk = true;
-        MOZ_FALLTHROUGH;
+        [[fallthrough]];
       case 45:  // 0x2d: Sandy Bridge-EP
       case 62:  // 0x3e: Ivy Bridge-E
         // Supports package, cores, RAM.
diff --git a/uriloader/exthandler/nsExternalHelperAppService.cpp b/uriloader/exthandler/nsExternalHelperAppService.cpp
index 698090718f..9112b67329 100644
--- a/uriloader/exthandler/nsExternalHelperAppService.cpp
+++ b/uriloader/exthandler/nsExternalHelperAppService.cpp
@@ -1794,7 +1794,7 @@ void nsExternalAppHandler::SendStatusChange(ErrorType type, nsresult rv, nsIRequ
           msgId.AssignLiteral("helperAppNotFound");
           break;
         }
-        MOZ_FALLTHROUGH;
+        [[fallthrough]];
 
     default:
         // Generic read/write/launch error message.
diff --git a/widget/GfxInfoBase.cpp b/widget/GfxInfoBase.cpp
index 9eefa6f372..a9658bbc06 100644
--- a/widget/GfxInfoBase.cpp
+++ b/widget/GfxInfoBase.cpp
@@ -984,7 +984,7 @@ GfxInfoBase::EvaluateDownloadedBlacklist(nsTArray<GfxDriverInfo>& aDriverInfo)
           } else {
             RemovePrefForDriverVersion();
           }
-          MOZ_FALLTHROUGH;
+          [[fallthrough]];
 
         case nsIGfxInfo::FEATURE_BLOCKED_MISMATCHED_VERSION:
         case nsIGfxInfo::FEATURE_BLOCKED_DEVICE:
diff --git a/widget/WidgetEventImpl.cpp b/widget/WidgetEventImpl.cpp
index 59c80672b1..f529ec7b1a 100644
--- a/widget/WidgetEventImpl.cpp
+++ b/widget/WidgetEventImpl.cpp
@@ -382,7 +382,7 @@ WidgetEvent::IsAllowedToDispatchDOMEvent() const
       if (mMessage == eMouseTouchDrag) {
         return false;
       }
-      MOZ_FALLTHROUGH;
+      [[fallthrough]];
     case ePointerEventClass:
       // We want synthesized mouse moves to cause mouseover and mouseout
       // DOM events (EventStateManager::PreHandleEvent), but not mousemove
diff --git a/widget/gtk/nsNativeThemeGTK.cpp b/widget/gtk/nsNativeThemeGTK.cpp
index cb4e7debf2..cfb9ef7eda 100644
--- a/widget/gtk/nsNativeThemeGTK.cpp
+++ b/widget/gtk/nsNativeThemeGTK.cpp
@@ -1306,7 +1306,7 @@ nsNativeThemeGTK::GetWidgetBorder(nsDeviceContext* aContext, nsIFrame* aFrame,
     // will need to fall through and use the default case as before.
     if (IsRegularMenuItem(aFrame))
       break;
-    MOZ_FALLTHROUGH;
+    [[fallthrough]];
   default:
     {
       WidgetNodeType gtkWidgetType;
@@ -1847,7 +1847,7 @@ nsNativeThemeGTK::ThemeSupportsWidget(nsPresContext* aPresContext,
     if (aFrame && aFrame->GetWritingMode().IsVertical()) {
       return false;
     }
-    MOZ_FALLTHROUGH;
+    [[fallthrough]];
 
   case NS_THEME_BUTTON:
   case NS_THEME_BUTTON_FOCUS:
diff --git a/widget/windows/KeyboardLayout.cpp b/widget/windows/KeyboardLayout.cpp
index b4801fa19c..c7bb60a675 100644
--- a/widget/windows/KeyboardLayout.cpp
+++ b/widget/windows/KeyboardLayout.cpp
@@ -1890,7 +1890,7 @@ NativeKey::InitKeyEvent(WidgetKeyboardEvent& aKeyEvent,
       if (mCharMessageHasGone) {
         aKeyEvent.PreventDefaultBeforeDispatch();
       }
-      MOZ_FALLTHROUGH;
+      [[fallthrough]];
     case eKeyDownOnPlugin:
       aKeyEvent.mKeyCode = mDOMKeyCode;
       // Unique id for this keydown event and its associated keypress.
diff --git a/widget/windows/nsWindow.cpp b/widget/windows/nsWindow.cpp
index 295518ea26..c636d38700 100644
--- a/widget/windows/nsWindow.cpp
+++ b/widget/windows/nsWindow.cpp
@@ -3525,7 +3525,7 @@ void* nsWindow::GetNativeData(uint32_t aDataType)
       if (pseudoIMEContext) {
         return pseudoIMEContext;
       }
-      MOZ_FALLTHROUGH;
+      [[fallthrough]];
     }
     case NS_NATIVE_TSF_THREAD_MGR:
     case NS_NATIVE_TSF_CATEGORY_MGR:
@@ -7660,7 +7660,7 @@ nsWindow::DealWithPopups(HWND aWnd, UINT aMessage,
         // compatibility mouse events will do it instead.
         return false;
       }
-      MOZ_FALLTHROUGH;
+      [[fallthrough]];
     case WM_LBUTTONDOWN:
     case WM_RBUTTONDOWN:
     case WM_MBUTTONDOWN:
diff --git a/xpcom/base/nsDebugImpl.cpp b/xpcom/base/nsDebugImpl.cpp
index c4d4437a9b..48b7e8e018 100644
--- a/xpcom/base/nsDebugImpl.cpp
+++ b/xpcom/base/nsDebugImpl.cpp
@@ -402,7 +402,7 @@ NS_DebugBreak(uint32_t aSeverity, const char* aStr, const char* aExpr,
     case NS_ASSERT_STACK_AND_ABORT:
       nsTraceRefcnt::WalkTheStack(stderr);
       // Fall through to abort
-      MOZ_FALLTHROUGH;
+      [[fallthrough]];
 
     case NS_ASSERT_ABORT:
       Abort(buf.buffer);
diff --git a/xpcom/ds/nsCheapSets.h b/xpcom/ds/nsCheapSets.h
index c2375b85f2..1d5aa21a8f 100644
--- a/xpcom/ds/nsCheapSets.h
+++ b/xpcom/ds/nsCheapSets.h
@@ -134,7 +134,7 @@ nsCheapSet<EntryType>::Put(const KeyType aVal)
       mUnion.table = table;
       mState = MANY;
     }
-    MOZ_FALLTHROUGH;
+    [[fallthrough]];
 
     case MANY:
       mUnion.table->PutEntry(aVal);
diff --git a/xpcom/ds/nsPersistentProperties.cpp b/xpcom/ds/nsPersistentProperties.cpp
index 4bbaac905d..50295984ac 100644
--- a/xpcom/ds/nsPersistentProperties.cpp
+++ b/xpcom/ds/nsPersistentProperties.cpp
@@ -226,7 +226,7 @@ nsPropertiesParser::ParseValueCharacter(char16_t aChar, const char16_t* aCur,
             aTokenStart = aCur + 1;
             break;
           }
-          MOZ_FALLTHROUGH;
+          [[fallthrough]];
 
         case '\r':
           // we're done! We have a key and value
diff --git a/xpcom/ds/nsVariant.cpp b/xpcom/ds/nsVariant.cpp
index 7e797117ad..ffec2575f0 100644
--- a/xpcom/ds/nsVariant.cpp
+++ b/xpcom/ds/nsVariant.cpp
@@ -355,7 +355,7 @@ CloneArray(uint16_t aInType, const nsIID* aInIID,
       if (aOutIID) {
         *aOutIID = *aInIID;
       }
-      MOZ_FALLTHROUGH;
+      [[fallthrough]];
 
     case nsIDataType::VTYPE_INTERFACE: {
       memcpy(*aOutValue, aInValue, allocSize);
@@ -759,7 +759,7 @@ nsDiscriminatedUnion::ToString(nsACString& aOutString) const
     case nsIDataType::VTYPE_WSTRING_SIZE_IS:
     case nsIDataType::VTYPE_WCHAR:
       NS_ERROR("ToString being called for a string type - screwy logic!");
-      MOZ_FALLTHROUGH;
+      [[fallthrough]];
 
     // XXX We might want stringified versions of these... ???
 
diff --git a/xpcom/glue/nsTextFormatter.cpp b/xpcom/glue/nsTextFormatter.cpp
index 96cd85712d..1c3f60fb90 100644
--- a/xpcom/glue/nsTextFormatter.cpp
+++ b/xpcom/glue/nsTextFormatter.cpp
@@ -352,14 +352,14 @@ cvt_f(SprintfState* aState, double aDouble, int aWidth, int aPrec,
       break;
     case 'E':
       exp = 'E';
-      MOZ_FALLTHROUGH;
+      [[fallthrough]];
     case 'e':
       numdigits = aPrec + 1;
       mode = 2;
       break;
     case 'G':
       exp = 'E';
-      MOZ_FALLTHROUGH;
+      [[fallthrough]];
     case 'g':
       if (aPrec == 0) {
         aPrec = 1;
diff --git a/xpcom/io/nsWildCard.cpp b/xpcom/io/nsWildCard.cpp
index 1250a41618..18c58810fd 100644
--- a/xpcom/io/nsWildCard.cpp
+++ b/xpcom/io/nsWildCard.cpp
@@ -80,7 +80,7 @@ _valid_subexp(const T* aExpr, T aStop1, T aStop2)
           return INVALID_SXP;
         }
         ++tld;
-        MOZ_FALLTHROUGH;
+        [[fallthrough]];
       case '*':
       case '?':
       case '$':
@@ -396,7 +396,7 @@ _shexp_match(const T* aStr, const T* aExpr, bool aCaseInsensitive,
         return ABORTED;
       case '\\':
         ++y;
-        MOZ_FALLTHROUGH;
+        [[fallthrough]];
       default:
         if (aCaseInsensitive) {
           if (::upper(aStr[x]) != ::upper(aExpr[y])) {