I looked at your firefight history which is here:
http://www.bungie.net/stats/halo3/gamehistoryodst.aspx?player =Many pebbles
Preview:
Wednesday, September 01, 2010
Length: 01:28:23
Players: Many pebbles , Maximus Joshus
Sets, Rounds, Waves: 3, 3, 5
It is unlocked when you play on Firefight, on any mission, and pass the 4th set (the 12th round and 60th wave) on 4 player Heroic co-op Live. It is worth 25 Gamerpoints.
There's your problem. You played on Easy, and only with 2 people. Needs to be 4 players on Legendary. And you need to get passed the 4th set, not just to the 4th set.