Launch TB from Terminal with the -safe-mode switch.

http://kb.mozillazine.org/Safe_mode#Linux

But I have never heard of authentication working just in safe mode. What are your add-ons, and type of authentication? For gmail, OAuth2 is recommended.

That's just it. I don't have any. I just use the standard flatpack but in safe mode the calendar feature doesn't work so that's probably an add-on. I have no idea why it would bork the authentication.

Does it authenticate in normal mode if Lightning is disabled?

Nope. Only safe mode works

I've also tried the standard ubuntu install package and it doesn't authenticate either.

I'll add that the standard package doesn't recognize any emails after December 2019 or so. I assume there was some change in encryption then that didn't carry over.

Again, what is the authentication type? The kb article states what happens in safe mode. Check if hardware acceleration is enabled in normal mode, and do a 'Reset toolbars and controls' from the safe mode prompt. See if you have a userChrome.css file. Add the account in a new profile, created from Help/Troubleshooting, about:profiles. What's your version of TB? It must be 60.9.1 or 68.5 at least to handle gmail's OAuth2.

TB is 68.6.0. No Chrome.css or even chrome* according to ~$ locate...

OAuth2 was working fine until a few weeks ago and when it failed I went to "safe mode". Not a big deal but there was a kernel update that req'd a reboot recently. Still, it's a nagging itch.

As an fyi, the current command to start TB is as follows: "/usr/bin/flatpak run --branch=stable --arch=x86_64 --command=thunderbird --file-forwarding org.mozilla.Thunderbird @@u %u @@"

I've no idea why you need to run with that command line, but it very likely is the reason you can't run TB except in safe mode.

That command line is from the Mint flatpak installation. If I use the normal TB for the Mint distribution it won't authenticate either and as I mentioned won't recognize any emails after December even though it is the same TB version. That uses the command "thunderbird %u".

So for the Mint flatpack I was able to use the following command to automatically start in safe mode. "/usr/bin/flatpak run --branch=stable --arch=x86_64 --command=thunderbird --file-forwarding org.mozilla.Thunderbird @@u --safe-mode @@"

Thanks for the help. I'm going to try restoring my profile from a backup to see if anything got messed up at some point. I do recall catfish locking up the system and having to do a hard reboot about the same time this issue started so it's possible some file got lunched.