I do not know and neither does anyone else if you do not know it. Only the person who set it will even know it. See https://support.mozilla.org/en-US/kb/protect-your-thunderbird-passwords-primary-password If you mean your mail account password, rather than a Thunderbird primary password, you will need to consult your email account provider. Thunderbird generally stores passwords in the password manager, but it does not set them and it does not store them for oAuth authenticated mail accounts at all. See https://support.mozilla.org/en-US/kb/password-manager-remember-delete-change-tb