If my car caught on fire, I would get everyone out and not try to extinguish it. After I (we) are at a safe distance, I would call 911.
a) You don't risk getting burned trying to put it out
b) Do you really want to have what your going to be left with...a car with (probably) extensive smoke damage, wiring damage, paint damage, and extinguisher residue? This is what insurance is for, and once a fire starts, you probably want it to be a good one.