|The Adas...Never leave the Normandy without it.|
In essence, the Firefight DLC adds 4 weapons that were previously only available in multiplayer (the Harrier, Geth Plasma SMG, Reegar Carbine, and Krysae Sniper Rifle), one PC exclusive (the Indra), and two new weapons only for single player (Adas Anti Synthetic Rifle and Blood Pack Punisher SMG).
Of the new weapons, the most exciting to me are the two new additions. The Blood Pack Punisher is a good general purpose up close demolisher that fires from two barrels, the top being conventional, and the bottom shooting an AP round for each burst. The Adas, while not easy to say, is an electrical weapon that would have brought a tear to Tesla's eye. It is great against shields and synthetics, of course, but also boast splash damage. Combined with an Armor Piercing mod and Warp Ammo, it's a regular squad buster.