If you are talking about the password for your email account, Thunderbirdhas nothing to do with that and is not going to send you anything. I am not sure where you even found a link that you thought would work for that. Your email password is administered with your provider. Go to their email help page and look for a link to reset your password.

T have forgotten a password of an existing for years now account and I want to reset it. Not the master password. Thanks