Using OAUTH2 has been an option for several years and now it will soon be mandatory for Gmail. TB has that option and it's rather straightforward. You need to change authorization by going to tools>account settings>server settings and changing authentication method to OAUTH2 and then going to tools>preferences>privacy&security and remove the gmail passwords - highlight account, rightclick and remove password. After that, shut down TB and restart. You will be presented with a menu to log into gmail to grant access to TB. Once done, life should resume.

Thanks David.

My wife uses TB for her Gmail account. Will this still work with POP3 after 30 May or must she switch to IMAP? And will all her archived emails still be there (they are actually stored in a file named Received and not in the TB archive facility)?

Should be no need to change. It is only access that needs to be in OAUTH2. I am not expecting any changes to accounts.

I followed your instructions, but kept getting stuck on the Google sign in window. I even changed my gmail password, removed gmail from TB, and tried to add it back again. But on the TB/Account Setup tab, after hitting Done, I got a "Checking password... Unable to log in at server. Probably wrong configuration, username or password" message. And, I also got the Google sign in window with my gmail address filled in. I checked and double checked everything, but no joy. Any ideas?

Concerning the answer given by david:

I do not find any global menu entry "//Tools>Account Settings>server". However, I have a global menu entry "//Account Settings", and there the different named servers and thereunder their settings. The menu entry "Preferences" also is not found unter "//Tools/Preferences", but under "//Preferences".

Beside from that, i am stuck at the login page where i have to put in my email address. After putting it in and pressing "weiter" (german, not changeable, because the browser page is completely locked, right click or debugging made impossible), the same login page shows up again and empty. After filling it in ... loop. Endless.

So, obviously, this thing does not work for me. Maybe because i have a german account. Maybe because of the devil knows what. Maybe Google should postpone this login policy change as long as they don't get it functioning? Maybe they should consult with the Thunderbird team?

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Update: You should add that both the receiver as well as the sender channel need to be switched. Despite that, it is still not functioning.

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After changing it back to "normal password", it works again.

Is there anything that must be switched on the server side? Do i have to login to my google account via web access and do i have to change something there?

Thanks David, worked fine for me including the location of the changes required.

White-Gandalf: I have encountered exactly the same problem as you, so I think that eliminates having a German account as the cause.

I described it here:

https://support.mozilla.org/en-US/questions/1369658