Thunderbird is a mail client, so it stores passwords for mail accounts, but that is as far as it goes. The resetting, allocation and just about everything else is managed by the mail provider. That is usually the company whose name appears after the @ in your email address. See https://support.mozilla.org/en-US/kb/password-manager-remember-delete-change-tb There have been some significant changes with some mail providers in the past few months. Who supplies your email? It may not just be an issue with a password. Note that if Thunderbird has just updated to a new version the issue is probably related to your antivirus product of choice blocking the new version.