openDialog("chrome://pippki/content/resetpassword.xhtml")    and press reset button. Then you will have to reenter all account passwords. 

Are you referring to your primary password, the optional one to start Thunderbird? Or are you referring to the password for an email account? If it's for a primary password, the steps are: click tools>developer tools>error console then enter openDialog("chrome://pippki/content/resetpassword.xhtml") and press reset button. Then you will have to reenter all account passwords. If the lost password is for an email account, you need to reset that at the online email account.