If you forgot the primary password, you can reset Thunderbird, but you lose all account passwords in doing that, as a security precaution. You would then need to reenter the various account passwords.

Choose Tools -> Developer Tools > Error Console, paste the expression:

  openDialog("chrome://pippki/content/resetpassword.xhtml")  
  and press the Evaluate button. That will open a dialog asking you if you want to reset your password. If Evaluate is not seen, press enter key. When prompted, press the Reset button. 

https://support.mozilla.org/en-US/kb/protect-your-thunderbird-passwords-primary-password#w_resetting-your-primary-password-if-you-have-forgotten-it

Ich habe das gleiche Problem: Warum gibt es keinen Weg, das Haupt-Passwort zu ändern, indem in dem angebotenen Fenster die Option "Passwort vergessen" angeboten wird?

Thunderbird cannot do that because Thunderbird is not the email address provider, only the agent. Thunderbird only knows what the user provides to it.