A gametype should be added to forge where you can make a firefight map with Phantom spawns, non-dropship enemy spawns, all of the regular forge options like human spawns, weapons and vehicles. You could then go into custom games and select the firefight game type to play locally or online with friends. This would eliminate the need for a whole lobby devoted to firefight. Players should be able to decide how many enemies spawn at a time in the properties of the enemy spawn object. I would also like to be able to choose what type of enemy spawns, be it Flood, Covenant, or Prometheans.
If there is concern that players would cheat challenges or achievements, the game could just check that the map was not a default map so it wouldn't award achievements or challenges. I am positive that there are hundreds if not thousands of players in the community that think this Forgeable Firefight is a great idea. I would prefer this to Spartan Ops any day and I think that goes for most of the community as well.