Hi, do you mean here on the support forum, or on your Firefox Account, the account used for the Sync service? On this forum, you can have a display name different from the username you use to log in, but I don't think you can change the username itself. At the top of the page here, your username should have a little triangle next to it indicating a menu. When you hover there, you should see an EDIT PROFILE option. On a Firefox account, your email address is your username and you can't change your email address. However, you can assign a display name. From the Options page, Sync panel, you can click Manage Account, or go to https://accounts.firefox.com/signin -- after signing in, you'll see a panel similar to the attached where you can change your display name. Or did you mean some other account?