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I have this same problem:

   Name: Thunderbird
   Version: 143.0.1
   Build ID: 20250923124009
   Distribution ID:

   Update Channel: release
   User Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:143.0) Gecko/20100101 Thunderbird/143.0.1
   OS: Windows_NT 10.0 22631
   OS Theme:

Hello

Thunderbird and Gmail. https://support.mozilla.org/en-US/kb/thunderbird-and-gmail#w_updating-a-gmail-account-to-use-oauth

Take a look at

Change the authentication method in Account Settings:

Change the Incoming authentication method:

I had a similar problem about two months ago when Outlook on my laptop lost access to my email. I never could overcome it and installed Thunderbird to recover. The Thunderbird installation went slick - no problems at all and all of my unread emails were downloaded when I finished. This problem began when I upgraded my cell phone. I had to change my password before I could use Gmail on the new phone. I never could update the password in Thunderbird despite countless searches for an answer. I've tried uninstalling and reinstalling Thunderbird but the problem remains. I'm reading my Gmail now using the Firefox browser.

Every time Thunderbird tries to get my mail, the screen pops up asking me "Sign in" to continue. When I tap the "Next" button, I get the "Google 400" error which is no help at all. I've attached the server settings that I have now.

TB worked so smoothly when I first installed it and I can't understand why I can't find a way to fix this problem now.

Thanks for your help!

Hello

Cookies ? Accept cookies from sites. https://support.mozilla.org/en-US/kb/thunderbird-and-gmail#w_updating-a-gmail-account-to-use-oauth

That was it! I had to check the box to accept cookies from sites! I'm downloading mail from May 13, 2024! It's going to take days to bring it up to date and get to the latest messages. I can only download between 270-285 messages at a time.

Actually, I just ran across the fix in a help page before I saw your tip. All's well that ends well. I'd better make a note of this. I try to be as private as I can. I don't remember clearing that checkbox but things are working now.

Thank you very much for your help!