When I go to Halo 3: ODST on MCC, I can’t find Flood Firefight mode. Isn’t it out?
Flood Firefight was delayed because of various issues, most notably with the AI. It’ll be released in an update some time this year.
I agree, they said that it would be released soon after the flight, which was 4 months ago, so, what gives? It’s been an eternity, and all 343i seem to care about is Infinite at the moment, they seem to be leaving MCC to die 
MCC can’t die. Not now at least.
I’d rather wait a while for flood firefight to release proper than have it come out half baked.
The flood AI in the flight was downright awful, sometimes they were idle in the spawn-room meaning you couldn’t kill them. Other times they simply were not hostile. Some maps they knew how to navigate, other maps they got stuck. And I never saw any of the little infection forms.
MCC still has a few content updates to release yet. All the games still need to have added modding tools, customs and file browsers still need expansion. While the focus right now is understandably on Infinite, MCC still has more to come.