Have you set up your Gmail account to accept Third Party Apps?

Yes and it stills stuck w/400 error!

There seems to be lots of different answers to this problem. You could try going to Hamburger icon (top right)/Settings/Search for Cookies. Turn Cookies on (if off) and retry. https://support.mozilla.org/en-US/questions/1441976

I had this problem also. The solution was to enable cookies in Thunderbird:

 Click ≡ > Settings.
 Select Privacy & Security.
 Under Web Content section, tick Accept cookies from sites.

You may need to set an exception:

 Click Exceptions… button.
 Add an exception for https://accounts.google.com
 Click Allow.
 Click Save Changes.

Exit and restart Thunderbird and then adding GMail works.

I got the solution from this helpful article (the section on OAuth): https://support.mozilla.org/en-US/kb/thunderbird-and-gmail