I bet they won't make anything like as much money with Firefly than they would have with Marvel Heroic though.
The so-called 'Big Six' licenses, according to RPG historian Shannon Appelcline, are:
Lord of the Rings
Harry Potter
Star Wars
Star Trek
Marvel
DC Comics
Harry Potter won't be happening any time soon due to JK Rowling's aversion to the rpg hobby (she famously tried to deny that she was a fantasy writer, as she thought it was embarrassing to her social circles, so she must view RPGs as the Antichrist of good taste...).
The rest have been taken up by various companies over the years. Personally I would add Doctor Who to that list these days, and maybe Game of Thrones as both are phenomenally successful TV franchises currently.
I really don't know what else to add to the list that truly qualifies as being as big. Avatar could be, I suppose, if it grinds out a few sequels. Top Gear (wherein you play reactionary, politically incorrect, middle aged men who have a puerile fascination for fast cars and hate the environment).