let's see...
single window (so that a new windows open in tabs instead of new windows)
toolbar enhancements (provides extra toolbar buttons like font increase/decrease)
download status bar (allows pauses/stop and sits conveniently in a status bar at the bottom of firefox indicating every download)
duplicate tab (like when u open a new window in explorer, it duplicates what was in the window u opened the new one from)
super DragAndGo (just highlight urls or links in a webpage and drag them anywhere empty to open a new tab w/ that link)
quicknote (quickly make notes in firefox in a sidebar, floating window, or new tab)
mozilla calendar