A fire extinguisher is somewhat useless...what are you going to do if your engine catches on fire - get out of the car, get your fire extinguisher, open the hood and put out the fire? No, you'd try and get as far from the car as possible.

If you're getting a fire extinguisher just for the "damn, my 106hp DX is too dangerous to drive around in so I need one of these" effect, then fine, but otherwise its a waste of time, money, effort, and is more dangerous than not having one.