Typed in wrong @domain; how change?

As above; Mozilla Firefox wont allow me to back up and change that which they're calling my error (the domain of my REAL email). I've tried all sorts (I think) of changing it to @mozilla firefox. Absolutely no luck; they refuse to let me get rid of @charter.net even though that's exactly what would correct this (maybe). I mean, that's where they're showing a blue field with wording to the effect that that line is wrong.