The puppet masters at mozilla support fixed this problem with the following, just in case anyone else needs International Rescue:
"I think you might have problems having normal password authentication for the incoming server and OAuth2 authentication for the outgoing server. It probably has to...
Any IT experts out there?
My Thunderbird email client stopped being able to download gmail a few days ago. I tried reinstalling, checked it was latest version, searched Google and Mozilla help but to no avail.
The error message I initially received was: Sending of password for user...