When they first announced Halo 4 was going to be Firefight-less, I was really disappointed. But as more and more details about Spartan Ops came out, and even having the chance to play a mission, I have to say that I'm glad they did this over Firefight.
Spartan Ops is a lot like Firefight and Invasion mixed together with a story built around it.
Oh yeah, this one was definitely due to a different approach. I started it at like 6 in the morning and finished about an hour and a half later- it was a request that a friend made. He actually requested it a long time ago (it's him and his brother as Spartans, basically), and I finally had a chance to do it.