edit firefox saved form?

I'm guessing you mean like, for forms with Name, Address, etc fields and you click and there's a dropdown list of things you've typed previously.

Let's say you've typed Alice, Bob, and Dave in the first name field in the past. You click Name and down comes Alice, Bob, Dave. Dave's not here, man. So you highlight Dave with the mouse cursor or select it with the arrow keys, hold shift, and hit delete. This removes Dave from the saved entries for that field name because, as we all know, Dave's not here, man.