What makes Firefight so much fun is precisely the fact that you can't sit back and wipe out all the enemies for hours on end from long range. It's not about "waping" aliens, easy mode style. That would be simple and samey and boring inside of Firefight's mission spaces. Instead Firefight, among other things, is about:
"The rush of being invaded by dozens of enemies at once as you huddle to regain your shields (I know you don't have shields, but you have something similar)... I don't think I ever felt anything close to this in campaign, but I experienced it within the first six minutes playing Firefight."
And yeah, sometimes you might *gasp* have to make due without the best weapons in the game and rely on your wits and cunning to make due. I know that might be a losing proposition for some of you (<3), but trust me, it's super satisfying in a way it never could be if everyone was simply picking off the Brute packs before they could get within a hundred yards of your position.