Post in https://support.mozilla.com/en-US/forums/contributors and a Admin may then see it and be able to then change it to a name you want.

Thanks James

Use a fake email address every time you forget your user name. I know it's idiotic, but there's no other way. You can't retrieve it because Firefox only allows you to reset your password, and there's no record of your user name in any of FF's emails. Even FF's built-in password storage feature only stores your password and associated email address, which is kind of pointless. The catch 22 is that you can't request any support unless you're already logged in.