In Halo 3: ODST for the Xbox 360, there are several Firefight missions you can unlock for some added gameplay.
Alpha Site Firefight – Complete Tayari Plaza in the campaign on normal difficulty or higher
Chasm Ten Firefight – Complete Data Hive in the campaign on any difficulty
Last Exit Firefight – Complete Coastal Highway in the campaign on any difficulty
Lost Platoon Firefight – Complete Uplift Reserve in the campaign on any difficulty
