I have fond memories of our Halo 3 Firefight nights on Xbox Live. We had huge community participation and hours upon hours and waves upon waves of Covenant to defeat. But when Halo: The Master Chief Collection finally added Halo 3: ODST to its assortment of games, it neglected to add the specific flavor of Firefight we loved.
While Halo: MCC does offer Firefight today, it is actually the version that was part of Halo Reach, which was also just recently released as an expansion to MCC. The biggest difference between the two in terms of gameplay - in ODST you play as an ODST, not a Spartan - making the waves more difficult and dynamic. It is a more stripped down version of Firefight than what is found in Reach, but it just feels more "pure."
It sounds like ODST Firefight will be coming later this summer to the entire collection on both PC and Xbox One. We'll be ready.
Source: Trueachievements.com