Don't buy those dinky little ones they sell in colors, the ones ricers put on their a pillar to look cool. You'd be lucky to put out a cigarette with those.
I have two in my car, one under the drivers seat, and one in the hatch. I used to have one, after my sisters car fire, I bought another. I also mounted one to the side of the center console of her new car.
I also put quick disconnects on the battery. Flip a lever, and its dead. Figure anything fuel or oil related will go out with the extinguishers and anything electrical will go out with the quick disconnect.
Almost had a fire in the TR once. Had an injector O ring break, and the injector started spurting fuel all over the intake manifold. Soon as I smelled gas I pulled over and put it out.
Every car should have one. Period. If you dont have to use it, someday maybe someone else will. I've come up on a crash and had one in my hands until I figured out the guy was ok and we could get him out.