UPDATE: Issue resolved.

I went to the Firebird Toolbar > Help > Troubleshooting Information. Clicked the "Clear startup cache..." button. I was then able to readd my Gmail and access the account.

Hope this helps someone.

I have had the same problem since upgrading to Supernova. I have 3 Gmail accounts, all were accessed from TBird. One then failed when changed to Supernova, so I deleted the account from TBird, expecting to set it up again immediately. I have tried MANY times without success and get a 400 error from Google. I tried the idea of clearing the startup cache (from Thunderbird, not Firebird! I think that was probably a typo!) but that didn't work either. In case it makes any difference, I want to set it up as POP, not IMAP. If I were to try setting it up in IMAP, would that like work better? Then could I change it back to POP? Hope someone can help

I have two GMail accounts. I was unable to set up either as POP through TBird. GMail does not support POP/SMTP.

I'm afraid that you can only use IMAPI with GMail — at least as far as I know and has been my experience. For that reason (and possible archiving issues and privacy issues[!!]), my primary provider is AOL (formerly Verizon) — with whom I'm able to use POP/SMTP through TBird.