What are the campaign and firefight modes in halo 3 odst?

I've never played, right now I'm just going through the solo game. Is campaign where you play through the game with a friend?What are the campaign and firefight modes in halo 3 odst?
firefight is where you just kill evil stuff for achievements and points and campaign is same as solo but you can replay missions you already finished by using mission select. you can do multiplaye and live for campaign, but you dont get any achievements in multiplayer.What are the campaign and firefight modes in halo 3 odst?
You can play both firefight and campaign with up to three of your friends or by yourself good luck my fellow ODST
well i dont have it but am sure its like specil ops in mw2 so dont take my info i havent got it yet

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