In addition to the big two extensions that almost all developers who work in Firefox have installed (i.e. Firebug and Web Developer), another very useful...
Firefox has a nasty habit of returning element background colors and colors in general in the format rgb(xxx,yyy,zzz) as opposed to the normal #xxyyzz.
You seriously learn something new every day. I only just found out about keyboard shortcuts to enhance tabbed browsing, something that has been a feature...