Hoo boy, I have a lot of thoughts on the Flood in ODST Firefight. The flood themselves are really underwhelming. They seem to be even weaker individually than in Halo 3 (and they were already too weak in that game), going down with very very little effort. They don't chase or pursue you, and just like in Halo 3, they die in a single melee hit, making them really easy for a game mode like firefight where you can just continuously punch them all to death as they slowly and helplessly approach you. It also doesn't help that very few of them spawn at a time, and there don't seem to be large hordes. They don't screech or yell, they hardly make any sounds at all. They kind of just meander around the map until you get close to them where they'll try to punch you. They never spawn with weapons, and their pathfinding seems to be really bad. They can't navigate around basic obstacles and they don't even notice you shooting them sometimes. Often they just won't leave their spawn rooms, even if you shoot them. So you have to kill the flood who actually entered the arena, and then go to all of the enemy spawn locations to finish off the brain dead ones.
Overall, we need major improvements. I didn't even get to see pure forms in action because I couldn't get them to spawn. For one, we need the flood to be more resilient across the board. Even on legendary, their health is pitiful, even compared to Halo 3 where they were already pathetic. Have them resist melee by a bit, too. The flood are supposed to put pressure on the player, so having them instantly die to a melee attack is really silly. And please please please give all Elite Combat Forms energy shields, and give them all the shield flare effect that the Brutes in ODST have. Doing this would give plasma weapons a solid role against the flood in firefight, and the shield flare effect is essential for player feedback.
Next, they need to be more competent and aggressive. No more meandering. They need to aggressively hunt the player and swarm when they can. They should leap, charge, flank, and they should almost always be on the move. I'd like to see tank forms made more aggressive than base Halo 3 as well, similar to what Ruby of Blue did with his Halo 3 Rebalance mod. Tank forms just kinda slow way down when they get close to you, and that just isn't going to cut it for firefight. Stalkers should also be made aggressive if you plan to have them in. They should try to flank and attack the player from behind since they aren't being used as a retreat class anymore.
Carrier forms need to be made faster in ODST Firefight than in vanilla Halo 3 and their explosion needs to be more potent. Give it a bit more range and make it deal a bit more damage and knockback. Once again, the flood is a class of pressure units, so they need to actually put pressure on the player to be threatening. Infection forms should also be made a little faster, a lot more aggressive, and their chain pop really needs to be reduced. Infection forms are hardly a threat when entire swarms of them can be instantly dealt with by killing just one of them with any weapon. They should also have the ability to scale walls at will, and they should swarm the player and other enemies by default.
Combat forms need to spawn with weapons more often than not, and they need to use them slightly more aggressively than they do in Halo 3. The reason they're so pathetic right now is that they aren't aggressive and don't swarm the player, they die very quickly to just about all types of damage, and they have no range option. These issues should all be addressed.
Skulls should also interact with flood as well as covenant. Let me explain how. Catch could make carrier forms spawn more infection forms at a time and have a more potent explosion. Tough Luck could cause combat forms and pure forms to actually dodge explosives/heavy projectiles and avoid grenades. Thunderstorm could make combat forms appear with more powerful weapons, as well as elite combat forms spawn as higher ranks with more resistant shields. Tilt could make flood forms' melees more dangerous and it could make their firing patterns more aggressive, but increase the damage headshot weapons do to pure forms, bullets do to all flood, and plasma does to specifically elite combat forms. Mythic is self explanatory.
Flood forms should have the option of spawning with active camouflage, maybe even make a squad type dedicated to cloaked combat forms.
Last thing. Give flood forms their sounds back. Have them do their classic roar/scream when a flood squad spawns, and give the forms themselves the ability to scream, roar,,etc. Sound is one of the most important aspects of the flood and it's completely missing here.