Many years ago in a time before Deloreans where built, I use to recharge dry chemical fire extinguishers. The dry chemical that was used and I believe is still used, is basically Baking soda or Sodium Bicarbonate, they add something to it to help keep it from absorbing moister and clumping. The propellant is dry nitrogen (if you used compressed air from a standard air compressor it would have too much moisture and would clump the contents of the extinguisher). My point here is that other then making a mess I wouldn't think that the actual chemical from a fire extinguisher should not be particular detrimental to the interior of a car as far as from a chemical reaction standpoint. I'm almost surprised the Arm and Hammer people don't promote it as a way of keeping your burnt car smelling clean and fresh after a fire LOL.