I did enjoy Warzone Firefight (still play it, especially since TitanFall 2 is under constant DDOS attack, and Frontier Defense is by far the best Horde mode in my opinion, with Warzone Firefight my 2nd favorite), but it is interesting that you state "proper" firefight. One of the camplaints from the anti-343 crowd was the lack of a classic firefight mode i.e. without REQs, no bosses, waves of enemies.
Also, there are several modes new to Halo introduced by Halo 5. I am partial to Roaming King in particular, where the hill itself slowly moves, following a specific path through the maps layout. Castle Wars is also fun when you just want something super casual to play.