Additional: I tried the suggestion of not putting in the password initially and waiting to be prompted. Also tried turning off firewalls. No luck.

The forum reports you as posting from Windows 7. I would expect Windows 7 to have issues authenticating with gmail as they have changed to a process Windows 7 does not support. However you say you use Windows 11.

This popup that occurs asking for credentials. Does it look like a web page with an address at the top?

Two things before you get into this. If you have a VPN turn it off. Gmail does not like them. If you have an antivirus that thinks it should scan email and encrypted connections, disable it as well. It is truly frightening the amount of folk that have issue only while their antivirus is protecting them.

On a similar note you might want to check in Thunderbird's settings that cookies are allowed. They are by default but interfering software folk installs sometimes install like CCLeaner among others disables cookies and that prevents the oAuth authentication process from working at all.

Thanks for the response.

Well, I don't "use Win11", just happen to be helping out on this one. I'm hanging on to Win7 until it becomes unusable.

Yes, looks like a webpage popup. Header says "Enter credentials for xxxxxxxxxx@gmail.com on imap.gmail.com". URL is "https://accounts.google.com/o/oauth2/[long data sting after that]".

No VPN running and no third party programs like CCleaner loaded from what I can tell.

Tried turning off all security, still no dice. Cookies are enabled. As a non-Win11 user, the "S" mode is new to me; one suggestion I've had is to disable that and then use the email client that they wanted (Mailbird) instead of T-Bird which was picked because of it being on MS "approved" app list.