I only did the very first mission, but it felt more like a co-op campaign mission than Firefight. More like things like MW3's Spec Ops mode, but less linear.
I enjoyed it on my own, as it still feels like a single player portion, but with most things it'll be better with co-op. The AI guy driving my warthog was useless!
Edit: Just saw the second post, its very much doable solo, you've got respawns but i dunno if they're limited, i died about 3-4 times.