If it were that important, cars would come standard with one. As it is, the only road-going car I know of that comes with a fire extinguisher as standard is the Porsche Carrera GT3. And that's only because it's a thinly disguised track car, complete with roll cage and barely legal tires.
It doesn't hurt having a fire extinguisher in your car, but you have a much greater chance of dying because the fire extinguisher brained you in an accident than dying in a fire caused by said accident (Unless you go through the trouble of securely fastening it somewhere that is both out of the way AND easy to reach in case of a fire). Cars very rarely burst into flame. And when they do, you'll have ample time to get out. And in any case, if the car burns out your insurance will buy you a nice new one instead of having the damaged one fixed, which is nice.
FYI: most fires are caused by electrical faults. You can prevent this by ensuring that everything is correctly wired up and having the proper fuses installed. DO NOT bridge a fuse that keeps blowing. It's not the fuse that's at fault. Keep a bucket of sand in your garage (or wherever you park your car). It should suffice.