Forgot to mention. Thunderbird can read new email messages from the problem gmail account. That seems to indicate a successful login. Thunderbird still can't send emails under the problem gmail account, failing login.

I just wasted a full two hours running in circles trying to follow the recommendations of the "experts" above I know NOTHING of emails (and plan to stay ignorant)

The answer is in a reddit post: https://www.reddit.com/r/Thunderbird/comments/v7ziud/how_i_fixed_oauth_problems_in_a_tb_profile_with_2/ "The gmail popups won't let me enter passwords... it just refreshes over and over with the email account box. Yes the accounts are still valid and working."

Go to Thunderbird -> Preferences -> Privacy and Security In the Web Content section:

ACCEPT ALL COOKIES - it is as yet unclear whether should be only all third party etc. and how long to keep them - can one be minimalist with these cookies

This IMMEDIATELY worked and all of my mail since June 6th downloaded

After repeated inspection and comparison of account settings for my working gmail account and my non-working account, I finally found a difference. Fixed it, and now both accounts are working.

The trick was in the top most tab, "Account Settings", then at the bottom of that page in "Edit SMTP server...", the authentication method was set to "Normal Password" for the problem account. I changed that to "OAuth2" to match the working account. Then, all was good.

The problem account is one I use regularly to receive email, but rarely use it to send. So, I'm not sure how long it had been broken. Thus, I'm not sure what broke it. But I am sure it wasn't me, as I've never been into those settings before. I can only suspect that a recent Thunderbird update was the culprit. Still seems strange that the account could receive but not send.

I don't understand most of the email terminology (SSL/TLS, OAuth2, NTLM, Kerberos, Ports), or why Authentication Method OAuth2 appears in both Account Settings and Server Settings.

Glad you found the answer to yours, seems like Thunderbird don't bother giving support. I've donated to them a few times too and we get nothing at all for our money. Time to find a new email client methinks.

There are several questions regarding same issue as you experience and I've been trying to get to all of them, but please understand this forum is manned by other users in an umpaid voluntary capacity offering what time they can and most help is done by only a handful of people. I'm sorry I did not get to you sooner, but I'm glad you got it sorted.

bschre022's answer would never be of any use because you are already able to access server to receive mail so it is obvious from the good info you supplied that cookies were not the problem.

re :why Authentication Method OAuth2 appears in both Account Settings and Server Settings.

The selecting of account name 'Account Settings' section deals with outgoing messages that use the smtp server and indicates what should be used when you Write emails to send and which outgoing server to use. The 'Server Settings' section relates to the imap server for accessing server to download emails from server. At the bottom of the left pane is 'Outgoing Server (SMTP)' and this lists all the outgoing server. You can add and edit in this location.