The ProtonMail Bridge is an application for paid users that runs on your
computer in the background and seamlessly encrypts and decrypts your
mail as it enters and leaves your computer. It allows for full
integration of your ProtonMail account with any program that supports
IMAP and SMTP such as Microsoft Outlook, Mozilla Thunderbird and Apple
Mail.

Once you've installed and adder your account(s) to the Bridge you will
need to configure your email client.  The setup guide for Thunderbird
can be found here: https://protonmail.com/bridge/thunderbird