Thunderbird Gets a 400 Error Setting Up Gmail

Normally, setting up a Gmail account in Thunderbird is a breeze. It auto-detects all your settings, pops up a log-in window, you allow Thunderbird access, and boom – you’re set! (I highly recommend Thunderbird, whatever email provider you’re using.)

But if you’ve been using this copy of Thunderbird for a while, you may have changed some settings, and if by some chance you’ve turned off cookies in the Privacy & Security settings, the Google login won’t work, and you’ll get a very generic error:

400. That’s an error.

The server cannot process the request because it is malformed. It should not be retried. That’s all we know.

Fortunately the fix is easy:

1. Go back into Privacy & Security.
2. Turn cookies back on. (It doesn’t even need third-party cookies!)
3. Set up the Gmail account.
4. Turn cookies off again. (If you want to.)

If you didn’t have cookies disabled, or if this didn’t fix it, this thread at SuperUser and the Thunderbird support forum are good places to look next.