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IDK if there's rank in firefight, but the second disk that comes with ODST is the halo multi plus three new maps. So yes, you will keep your rank and it will be the same multi. Check out the Halo 3 ODST project page.
Its the exact same multiplayer
Doubt that there will be a rank in firefight unless they incorporate somesort of matchmaking.As for the second disk, it will be they same as your halo 3 rank.
ive got a bullet with your name on it but i accidently used it on justin bieber
*facepalm* yes seriously how many times