When trying to add Gmail account in Mailbird you get the error message:

Clear your browser's cache and cookies. If that won't help try to use incognito mode or even try to change your default browser for the time of setting up OAuth.

You can change the browser to any of the installed on your OS using the following guide: https://support.google.com/chrome/answer/95417?co=GENIE.Platform%3DAndroid&hl=en.