have you updated your authentication method to oauth as required by Microsoft to allow ongoing connections?

Yes ,these are already enabled. We have 2 domains operational with same environment. One domain is working well.

2nd Domain has almost 500 systems with thunderbird client and mailbox in Exchange online. The issue is happening only in this domain and with selective users.

We tried configuring profile of a working mailbox in a system where the mailbox configured is not working. In this scenario, working mailbox continued to work along with the mailbox which was not working. So it doesnt seem to be issue with client as such. We tested the scenario otherway round. by configuring non working email in a working system where mail is working. There also the non working mailbox does not download emails, though the working email continued to download mails.

Note : We received an information from one of the other customer organization who uses thunderbird client with pop3 that for them as well the mails stopped downloading since last 2-3 days and they are also running behind microsoft for checking the same.

Please continue in https://support.mozilla.org/en-US/questions/1452037 so that we have one location for this issue.