If you thought dealing with one Chieftain was a pain, try dealing with a whole pack.
Firefight mode does more than simply pile on the enemies though, it also escalates the difficulty in more creative ways using the same skulls found in the Halo 3 campaign. As such, you might find the first round of play to be a breeze only to find that the next round of foes have more health or become more adept at dodging your grenades.
Bungie's Brian Jarrard informed us that something special happens whenever a group of players successfully completes a set, though he wouldn't elaborate on what it was. We don't know how many maps Firefight mode will have, though Jarrard told us that they are all pulled from sections of the main campaign. Once players complete a specific campaign section, it's unlocked for play in Firefight.
We got to try out two of the game's new weapons -- if there are more, Bungie's not saying -- including the new pistol and a silenced SMG. The silenced SMG performs exactly as you would expect it to, spraying enemies with lots of relatively ineffective bullets, though it does include a scope this time around. The pistol is a lot of fun -- Bungie's Joseph Staten called it "a beast" -- and features a nice scope and very fast semi-automatic fire. Oh, and it headshots grunts real nice.
The ODSTs themselves have some tricks of their own, most notably a special visor mode to highlight enemies and make them easier to pick off. This is especially useful for invisible enemies that show up in later waves. While ODSTs might have this slight advantage over Master Chief, it's important to remember that they have additional human frailties. That means they don't have regenerating shields (hence the health packs) and, yes, they take damage when falling long distances. We also noticed the significant difference in scale. Jackals, which look pretty small from the seven-foot-tall vantage of Master Chief, look much more intimidating through the eyes of a normal human ODST.
So, yeah, Firefight mode is Horde mode with a Halo twist. There are lots of bells and whistles that make it a different experience, but the core gameplay concept translates just fine in the Halo universe, and that's really all that matters.