Some ideas for Firefight maps (some are my own, some are just ones I've seen in similar threads)
1. All three positions where you find and defend the marines in the second level (called Halo) of Halo:CE. Especially the rockslide place, I'd love to see that remade.
2. The start of the level Outskirts from Halo 2. "We've got jackals in the courtyard", "Buggers. Coming over the rooftops", "Someone get a satchel on that gate", the blind skull, hunters. You know the place.
3. Ok so noones actually played it yet (other than Bungie and members of the press) but I reckon Longshore looks like it could be made into a great Firefight map.
4.Kalle90 suggested Zanzibar (post above) and I agree.
Also just as an aside someone said ODST takes place inbetween Halo 2 and Halo 3. Please do not misdirect. ODST takes place DURING the events of Halo 2