To begin with, Firefox doesn't need a User Name and / or Password to be able to use Firefox. That is an optional feature typically used for Sync, but also used for other Mozilla / Firefox "services". The '''user name''' for Sync services (your "Firefox Account"), is the email address that you used when you initially set up the Firefox Account. If you can't remember that email address, "you're up the creek without a paddle". The system can't send you an email if you can't recall the email address. Try this to view your Saved Logins on another device which is also using the Firefox Accounts system ... Type '''about:preferences#security''' in the address bar, then hit enter. Look under Logins -> Saved Logins for '''https://accounts/firefox.com''' -- '''''Username''''' field will display that email address.