CO2 is the unavoidable product of combustion and aerobic metabolism. We breath it out; produce it when we drive our cars, and indirectly produce more when we use electricity. It is extremely stable and absorbs strongly in the infared spectrum turning it into a greenhouse gas. It is colorless, odorless, and won't leave any residue. Its extremely unreactive, but not as inert as argon, lets say, so you can't use it for arc welding if you want a really clean and strong weld. However, for fire fighting, its basically there to flush away the oxygen and smother the fire by depriving it of oxygen. It is heavier than air and probably beneficial for this application but is probably not good if there are people crawling around on the floor during a fire.