You have to remember both the Firefox Account email address (which is the Username) for your Firefox Account AND the Password, too. Look at your Logins on another connected device; and if that was the only device that you had connected to Sync, you are using the Sync service other than was intended. Plus, if you Reset the Firefox Account password your saved data in Sync will be wiped from your account. Sync data is encrypted using the Password as part of the "encryption key", so a different Password will not work.