It would be used similar to how a waterbender uses mist, or how an earthbender uses dust. Using smoke would be a defensive maneuver. It would act as a shield or cover to the firebender using it. Additionally, it provides a silent offensive aspect, as smoke can be used to suffocate opponents. However, it is worth noting that firebenders would only be able to create the smoke, and only if they burn appropriate fuels. There is no way they could control the smoke.