If you discover that your Microsoft 365 / Exchange mail account password is no longer working in Thunderbird, please take note of this:

Microsoft is force rolling out deprecation of normal password authentication, so you'll need to switch to "Authentication Method: OAuth2" in the account's Server Settings.

This should resolve your issue.

You can read more about this change here: https://techcommunity.microsoft.com/t5/exchange-team-blog/basic-authentication-and-exchange-online-september-2021-update/ba-p/2772210