If you have forgotten the password for an email account, your best bet is to contact the email provider. If you are referring to the Primary password, the steps for that are below. This can be problematic, but usually works. This will also remove all account passwords:

Choose Tools -> Error Console, paste the expression: openDialog("chrome://pippki/content/resetpassword.xul") and press the Evaluate button. That will open a dialog asking you if you want to reset your password.