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  

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. More info here: https://support.mozilla.org/en-US/kb/protect-your-thunderbird-passwords-primary-password#w_resetting-your-primary-password-if-you-have-forgotten-it Your account passwords will all need to be reentered, as this process removes them from the profile.