It's not hard to see the appeal of the idea of the Firefight mode in Halo 3: ODST. If you follow Bungie's shooter roots back to Marathon, and to the PC shooter that really kicked off the modern era, Doom, you can see the start of it.
Doom didn't have discrete multiplayer 'levels' the way Marathon and Halo did. It had a series of Episodes, each broken down into Missions that comprised one map. Each map had a series of keys necessary to open a series of doors. The last door was the Exit and led to the next Mission. Between you and each key and each door were a number of demons to kill. You could tackle this challenge solo, or bring in some friends on a local network.
Of course, you could just as easily shoot your friends as the demons. You could also set up a game on any Mission map without any demons and just play deathmatch, or you could play a deathmatch game with continually respawning demons on it.
Marathon had a similar setting, an "Aliens" checkbox that put enemies from the campaign mode onto the multiplayer maps. So while it wasn't always referred to the same way, Marathon had all the current play modes: campaign solo, campaign co-op, multiplayer (deathmatch and objective) and deathmatch-with-aliens. That's essentially what Firefight is, except it's supposed to be more towards cooperative.
So we can now safely say that with the release of Halo 3: ODST, in combination with Halo 3's multiplayer mode, has finally brought all the features of a 1994 Mac shooter to Xbox Live.
I'm only half kidding.
