Here's a fun thing you can do in Halo 3's Campaign to get some "practice" in for Firefight. Set the difficulty to Legendary, and put on all skulls except for Iron, Blind, and Black eye. Then load up either:
1- Tsavo Highway, Rally Point Alpha
2- Crow's Nest start
Defending the base in Tsavo Highway from the huge packs of Brutes, and defending the hangar in Crows Nest's from a combination of Covenant Infantry, offer similar challenges to what you'll experience in Firefight.
For a bonus challenge, keep the number of deaths during the entire encounter under 15.