I dunno if Firefly could be brought back after Serenity. They kind of ended a load of storylines pretty finally, and missed out a load of stuff which probably should have gone before them and would have, if they had had a few more series to tell the story over but can't really be told now what with people dying and whatnot. (I'm trying not to spoil things for anyone who hasn't seen Firefly or Serenity yet.)
To me, Serenity is to Firefly what the Peacekeeper Wars were to Farscape, a way to finish things off as best as they could, even if it wasn't how they would have liked it to be done.