Firefight multiplayer game mode unlike Halo 3 mulitplayer has no unlockable armor “types”.  Players get to unlock Halo 3 ODST playable characters instead that wear variations of the ODST armor.  Well ok, Dare does wear the ODST Recon armor.  Like Halo 3 multiplayer the characters have no special bonus that alters gameplay.  The character armor is only asthetic in value.

Here is how you unlock the character armor:

Rookie – Default

Buck - Complete “Tayari Plaza” on Normal or higher.

Dare – Complete the campaign on Legendary.

Dutch – Complete “Uplift Reserve” on Normal or higher.

Mickey – Complete “Kizingo Boulevard” on Normal or higher.

Romeo – Complete NMPD HQ on Normal or higher.

Sgt. Johnson – Preoder Halo 3 ODST to unlock with code.