Well, as far as MMOFPS's go, there aren't many. But if you excuse me for a second, I'm going to fanboi out here for a second.
PlanetSide 2. Omahgod. It's a first person shooter with a massive, persistent world. Not just one and done like other FPS's. If you win a 'map' you have to be able to hold what you gained. Check it out. Oh, and did I mention it's free to play? 'Cause it is.
Ok, now that's out of my system. It's still up-and-coming, you can sign up on the list for the beta on their website. Link down there. All of the other suggestions are good shooters, by the way. Also maybe try Tribes: Ascend. It's got one of the funnest and most unique combat systems out there. Firefall is another game that fits what you're looking for. It's a third person shooter and is more of an MMO than any of the others. I think it's in beta still, though.
Tl;DR Planetside 2, FireFall, and Tribes:Ascend. Both free to play, PlanetSide isn't out yet, though and FireFall is still in beta.
I hope this helps.