UPDATE:

Although I though that I had fixed my wife's login issues with the actions above, two days later her account would no longer connect to gmail. No error message, just never would connect and download messages. My account was fine--but very slow.

Uninstalling TB and reinstalling (147.0.1) after a reboot, using default-release instead of esr, has so far fixed the problem. Still don't know how I ended up on the esr channel.

Password - that's the one you use to access the gmail webmail account. Ideally at least mix of upper and lower case, include numbers and use 12 various should be fine.

But Gmail uses Oauth2, so once set up Thunderbird will use the OAuth token when it commmunicates with server not the password.

I'm presuming you are using an IMAP connection, so let me know if you use Pop.

Firewall: Access Firewall - it might be the defaulton computer or controlled by an Anti-Virus prgram. Set up Thunderbird program as 'allowed' so nothing gets blocked.

• Confirm Thunderbird is set up as allowed program.

VPN: Check computer for a VPN. Some Anti-Virus products now offer a VPN and I have come across some peopl who did not realise one had been enabled. Switch off VPN (or create split tunnelling and set it up for Thunderbird to use). Gmail does not like VPN as it atempts to hide your IP address, so gmail decides you might be hacker so it auto blocks access.

• Confirm VPN is switched off

Localhost: Do you have anything using the localhost? People who manage websites may typically use eg: Apache Xampp. You need to switch of any program using the localhost whilst setting up gmail. You can enable it again afterwards. I had to switch off Apache Xampp.

• Confirm nothing is using localhost.

In Thunderbird do these checks:

• Settings > Privacy & Security
• Web Content
• Select 'Accept cookies from sites'
• Click on 'Exceptions' button
• Enter this : https://accounts.google.com
• Click on 'Allow
• Click on 'Save Changes'

Whilst in that area - scroll to Passwords section - I'm suggesting you clear everything so forcing a new token.

• click on 'Saved Passwords'
• If you see any mention of the gmail account username = gmail email address - select the relevant lines and click on 'Remove'
• Click on Close

IF you are not going to remove and recreate the GMAIL account: Exit Thunderbird - wait a few moments and then restart.

If you intend to recreate the removed gmail account then... I would switch off the following as it's still experimental.

• Settings > General
• Account Hub
• uncheck this checkbox: 'Create accounts in the new Account Hub'

IF you removed the account now create the mail account

• Menu app > New Account > Email
• Enter name
• Enter gmail email address
• Enter password - same one you use to access webmail
• Select checkbox to remember password.
• Click on 'Continue'

Thunderbird by default will look for the IMAP server settings. They should be:

• Server name: imap.gmail.com
• Port: 993
• Connection Security: SSL/TLS
• Authentication Method: OAuth2
• Username = full gmail email address
• Password is the same one you use to access webmail

Outgoing SMTP:

• Server name: smtp.gmail.com
• Port: 465
• Connection Security: SSL/TLS
• Authentication Method: OAuth2
• Username = full gmail email address
• Password is the same one you use to access webmail

Assuming all is ok click on 'Done'

Whether recreating he account or just checking settings and removed password....

You should then get the gmail pop up browser type window. It will ask you to confirm you really are you and the then ask if you want to allow Thunderbird access to server. Obviously you want to allow it.

Then an OAuth token gets created and stored in the same place as all other passwords.

If you are talking about a POP account then gmail can become a pain if there are too many emails still on the server. They basically stop the downloads.

This issue occurs because it's not possible for Pop accounts to put deleteed emails into the server Trash folder.

Access gmail webmail account. I do not know if you still have a load of emails int he Inbox, but if you access 'All Mail' it will have evreything - delete loads of old emails - remember if using a Pop account in Thunderbird, you already have a copy of all emails which you can also choose to backup.

I've also come across some people who discovered the Pop access had been disabled in the webmail account.

Thanks for all the very helpful information. My atomic bomb fix (completely uninstalling and reinstalling) is still working, and many of your suggestions came up when I searched for solutions before I did that. I tried the ones that were relevant.

Nevertheless, it is very nice to have them in one place and from an expert, so I am downloading and saving them for future reference.

The reason I posted and reposted was to learn if anyone else had seen anything like this affecting gmail. I have a related post about gmail slowing down lately when I access it through TB, and I'm trying to figure out whether it's just me, or a more general problem. (if it were just me, I'd have expected it to go away after the reinstall, but it didn't). Remember, I had not changed any settings on TB or my computer before all this happened.

Thanks again for your help.