Thunderbird v78 imap.gmail authentication failure
Once my Thunderbird account updated to version 78 I can no longer access my imap.gmail. I had no problems prior to the update.
I found another topic (archived 1 year ago) entitled 'imap.gmail/Thunderbird Authentication failure'. The solution is to reset all gmail account passwords through the browser (why?). Then delete all gmail account passwords stored in Thunderbird. Finally, change the server authentication method to OAuth2.
I don't know if that solution is still relevant, but I thought I would give it a try. I didn't get very far.
I cannot find a way to delete the gmail account passwords stored in Thunderbird without also deleting the usernames associated with those passwords. To preserve the usernames (indeed the entire email account information), I tried editing the password to a blank entry, but Thunderbird would not allow that. It seems that the record for the entire email account must be purged from the list. I browsed the Mozilla knowledge base and could not resolve this issue.
Isn't it sufficient to just change the server authentication method? Why is it necessary to delete the email account username/password entry associated with each gmail account? What a PITA Thunderbird has become.
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The solution is to reset all gmail account passwords through the browser (why?).
Why resetting all gmail account passwords or why resetting through the browser? The former shouldn't be needed unless there's a particular reason for you to change your Google account password. The latter should be obvious, this is your Google account password and not a Thunderbird password. </blockquote>
Then delete all gmail account passwords stored in Thunderbird.
With OAuth2 Thunderbird doesn't remember the actual Google account password but an OAuth2 authentication token. Therefore you can delete your Google account password Thunderbird has remembered. Sometimes authentication fails when there's an old authentication token Thunderbird has remembered. So it's recommended to delete those old entries if they exist. </blockquote>
Finally, change the server authentication method to OAuth2.
You'd definitely need to do that.
I cannot find a way to delete the gmail account passwords stored in Thunderbird without also deleting the usernames associated with those passwords.
At the top right of the Thunderbird window, click the menu button 
> Options > Privacy & Security > Passwords > Saved Passwords
Select the entry with the password you want to remove - click Remove
This will not remove the related email account.