This could be caused by Google not recognizing and blocking Thunderbird's default user agent; try toggling general.useragent.compatMode.firefox to true (which basically has TB emulate Firefox's user agent)

I am having the same difficulties, and it's giving me a headache. The only workaround was to change the authentication method to "Normal password", and enable the "Allow less secure apps" setting on the Gmail account.

And that's a major PITA since I have 40-ish Gmail accounts. (Why so many? I tend to separate my different concerns for privacy and security reasons.)