Follow [https://support.mozilla.org/en-US/kb/protect-your-thunderbird-passwords-master-password#w_removing-your-master-password-if-the-master-password-has-been-forgotten these instructions] if you're able to launch TB. If not, open the profile folder, located by default in: C:\Users\''username''\AppData\Roaming\Thunderbird\Profiles\xxxxxxxx.''profilename'' and delete or rename key4.db, cert9.db and logins.json. This will erase all account passwords.

What are the x's ?

The x are a random string of 8 numbers and characters, unique to each profile.