hii felton1373, You most certainly already have a Firefox account under that email address if you are receiving that message. You only need to remove the account that is currently associated with that email address in order to add it as a secondary email address to your current account. Simply use the desired email address to log in to the Firefox accounts page (the one that you use for Thunderbird). You can ask for a password reset if you don't remember it. Just erase it after you log in to that account. Following that, you ought to be able to log back into your current account and add that email address as a backup. Hope this was useful.