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. 

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