Make sure you choose 'OAuth2' as 'Authentication method'. Note, you will need to use your normal Google password with 'OAuth2', not an app password.

Changing to OAuth2 made no difference. I also tried changing the default browser from FF to Chrome. Tbird setup opens a signin page for gmail and when I type in my email and pw I get a page about enabling cookies. I have enabled cookies. I have cleared cache. I have opened a new window. All that stuff. Still get that cookies page. Then tbird setup says username or password is invalid.

The setup verification of my pw is not working and is blocking me from adding my gmail account. What do I do?

You have enabled IMAP and SMTP on the Googlemail website?

I also tried changing the default browser from FF to Chrome.

Why? That's completely unrelated.

Tbird setup opens a signin page for gmail

That's about right.

when I type in my email and pw I get a page about enabling cookies. I have enabled cookies.

You only need to allow cookies for https://accounts.google.com in Thunderbird, not in a browser.

Thank you. No where did I find anything about having to enable cookies in thunderbird. Somebody needs to explain that somewhere. You solved the problem. Once I enabled cookies in tbird--no problem. Setup worked perfectly.Thank you so much. I was going crazy. pat

I now have this problem as well, and it seems to have come out of the blue, suddenly today I have the "Login to server imap.googlemail.com failed." (buttons: "Retry", "Enter new password" & "Cancel"), none of which change anything, & I can't think of anything which might have caused it.

I have only one gmail account, which I've used Thunderbird for for years, & I can't find anything in Thunderbird about cookies, or indeed any menu link to any preferences.

Can anyone advise or tell me what information you need me to provide in order to be able to do so?

Matt-A,

you're posting to a solved thread. Please start a new thread for your problem. https://support.mozilla.org/en-US/questions/new