If this is an account password, you should contact your email provider. Or is this a Primary password that prevents starting thunderbird?

Primary password

It is the primary password

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

  openDialog("chrome://pippki/content/resetpassword.xhtml")  

1. then, 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.
2. When prompted, press the Reset button.

This erases all account passwords.