If you use Thunderbird as a client for Microsoft-owned accounts and have sign-in issues, especially recently, be sure to check our newly updated KB article on the topic. https://support.mozilla.org/en-US/kb/microsoft-oauth-authentication-and-thunderbird-202
We'll be keeping a close eye on these issues, and thanks to everyone who has both let us know about their problems and jumped in to help others. 😊 💙
