Firefight barely plays for me anyway.
It uses a different networking system from normal matchmaking, in order to accommodate the AI. This system is was obviously not developed for people with less than favorable connections who also can't upgrade because no one sells anything better in this damn place. Well one company was about to, but they went under and got bought out by another, bigger company who seems to not give a damn. Pricks.
Sorry, got kind of sidetracked there...
Anyway, whenever I play it(or campaign co-op), its dreadfully laggy usually to the point of unplayability. At best there's a 1/4 second delay in everything I do. At worst its like playing a game at 0 FPS.
Not sure what this has to do with your problem, but I needed to get that off my chest.