The Gmail signon stops working for me after a while. It results in a Blank window or a Temporary Error banner. The solution for me was to go to Preferences and delete the Cookies for mail.google.com which was in hundreds of MBs and then add mail.google.com to the Manage Exceptions list with Allow for Sessions only.
I didn't have this problem with older 78.0 esr versions of Firefox but have it with 86.0