You aren't allowed to use incendary weapons on people according to the UK rules of war, no idea where it's set down but that's what i was told. Maybe on vehicles it's different, i know you are allowed to use White phosphorous, a very nasty thing indeed, to put down a smoke screen, but you aren't allowed to use it as an anti personell weapon (think napalm but far worse).
FAE's or thermobaric weapons don't kill by burning, they kill by overpressure (blast) and by sucking all the air out of an area (Although you'd probably be dead from the overpressure anyway).