There is a giant new article about it in the months issue of Game Informer I got in today, there's no way to post it unless you just have the magazine your self but
THIS article pretty much sums up what was said in Game Informer. It appears they have done away completely with Firefight :/
It looks like Halo with classes, and loadout? Whaaat, they seemed to of changed a lot with the multiplayer system! Check out this video!
On the bright side it looks like the Ol' Battle Rifle is back! :D