the reason you see people playing firefight in third person is because when you re watch a film, you con only see the first person view of the player who saved the film. so when you see people in third person, theyre either trying to show off the enviroment or just dont have access to the frist person view
example; go fire up halo 3 play a game and then go to theatre and watch the game you just played, then switch to another player and click the right stick in, it wont work