Start with 2 step verification turned off. Or also try creating an app password for Thunderbird.

Check that cookies are enabled and that you have no software running on Localhost. Generally reports have included Apache. But anything that consumes stuff posted to localhost will cause issues.

Try deleting the files that are used to store passwords. This gmail change has seen a slew of folk upgrading from old versions of Thunderbird and the password storage is corrupted because they never had V68.5

Extracted from https://bugzilla.mozilla.org/show_bug.cgi?id=1768542#c6

Menu app icon > Help > More Troubleshooting Information Under Application Basics' - Profile Folders - click on 'Open Directory' (Linux) or 'Open Folder' (Windows) You will see window showing contents of profile name folder.

Exit Thunderbird now - this is important

Look for the following files and delete them.

   cert8.db - obsolete file
   key3.db - omelette file
   pkcs11.txt - this file may be missing as seems to be being reported in these cases - if you see it delete it
   secmod.db - obsolete file
   session.json

NOTE: do NOT delete the following:

   key4.db
   cert9.db
   logins.json

Start up Thunderbird pkcs11.txt and session.json get created. Oauth2 Authentication should now work.

Note if the first set of deletions do not work, remove the key4 and logins files as well.

Thank you very much for your quick response! I really appreciate it!

After going through your recommendations, I am still getting the notification, "Authentication failure while connecting to server imap.google.com", when I allow Thunderbird to access my Gmail account.

Here are the individual steps I took:

I turned off 2-step verification.

When I took my computer into a computer store on June 2nd to try to get past this problem, the technician created an app password for Thunderbird in Gmail, but that didn't change the situation. Google Gmail recognizes my current password and is ready to authorize access, but there is always an authentication failure.

Enhanced Tracking Protection was switched from Custom to Standard Protection.

I enabled Total Cookie Protection and reloaded all tabs. I couldn't tell that all tabs had be reloaded, so I closed and restarted Thunderbird.

I don't know how to check if software is running on Localhost and I didn't find any information on the web that would help me

When I entered 127.0.0.1 in my Thunderbird browser while trying to find local hosts, the file named changed to http://127.0.0.1/dashboard/ and a page was opened with the heading, "Welcome to XAMPP for Windows 7.0.8".

phpMyAdmin is running as a local host and three different local hosts are listed in the User accounts overview:

   User name    Host name   Password   Global privileges

1. Any localhost No USAGE 2. pma localhost No USAGE 3. root localhost No ALL PRIVILEGES

I haven’t saved any passwords in my Google Account yet, but I deleted a few I had in Chrome.

With regard to updating Thunderbird, Windows 10, etc., it is my practice to updates apps as updates become available. There has never been a gap between updates.

I found and deleted all of these files as recommended:

  cert8.db - obsolete file
  key3.db - omelette file
  pkcs11.txt
  secmod.db - obsolete file
  session.json

I noticed that there is a session.json.backup file and also files session-1.json through session-24.json in this folder.

Thunderbird is finally working for me!

Before sending my last response, I missed and didn't do your last suggestion: "Note if the first set of deletions do not work, remove the key4 and logins files as well". I deleted two logins files but wasn't able to delete key4 by accessing it through the Thunderbird Help process, since key4 was being used by Thunderbird. So I closed the app and deleted key4 directly from the profile file.

I also deleted XAMPP since it was using three localhosts and phpDesigner that was used in conjunction with XAMPP.

In the meantime, Windows issued a new update that I downloaded and installed.

I don't kow which of the above steps was was the key in getting Thunderbird to work, but it is now working very well!

Thank you again for all your help!