I'm afraid that may not work in Thunderbird. I'm surprised it even worked before, because the OAuth2 sign in pop up should be specific to an account that's been setup in Thunderbird. It should not allow changing accounts on the fly in the middle of the OAuth2 authentication process, thus bypassing the underlying account for which Thunderbird initiated the OAuth2 sign in process. I expect you to have setup your me@domain.com account in Thunderbird, separate from the shared@domain.com account, so that each account exists and authenticates separately in Thunderbird.

in TB I have a me@domain.com account, and it works. This is my personal O365 mailbox. Then I need to access some other shared mailboxes, that I did configure in TB as shared@domain.com username, changing the validating login in Oauth2 authentication. I need to tell to O365 that I'm me@doman.com and I want to access the mailbox shared@domain.com. I'm delegate to do this in remote server, of course, and I need to perform this kind of process that is ok. I canno authenticate me as shared@domain.com because that is not an "identity" but only a "resource".

Until TB 115 no problem.

I cannot understand from your reply how can I perform this goal.

So, you've already configured shared@domain.com in Thunderbird, but in order to authenticate, you need to use me@domain.com as the username, correct? In that case, Open Account Settings for shared@domain.com, then under Server Settings, set the User Name to me@domain.com\shared@domain.com.

This is what we did before Oauth2 with plain auth access to Exchange shared folder, and I'm not sure this makes sense in OAuth2.

I tried, it does not work. However if I insert anything with "\" in the login field (me@domain.com\shared@domain.com or also only "\shared@domain.com") the OAuth2 popup leaves to edit and change the login, so I can write me@domain.com and try to perform an access as I need. Still, after the authentication TB reports "authentication failed".

Remove the shared mailbox account from Thunderbird then add it afresh. The OAuth2 url for MS 365 email seems to have changed in TB 115. If that doesn't work, you may have to revert and stick to the previous version of TB. Another option is to have app passwords enabled on the remote server.

tried to remove and recreate the account, still the same. No way to authorize. This make me crazy, the OAUTH2 authentication was great and well working also for shared mailboxes, it may simple be as it was before.. :(

Any news about this? I'm forced to leave TB if this bug is not solved.. I cannot connect to most of my mailboxes!

You posted a follow up at https://support.mozilla.org/en-US/questions/1431515

If you are still seeing this problem please post a new topic.