I tried keeping a fire extinguisher. It's a pain to find a way to secure it in the cab, because it absolutely has to be secured in case of a crash. If you keep it in the trunk -- assuming you can have means of securing it -- it's not immediately accessible in case of emergency.

Also, halon is expensive, and a good one (with sufficient suppressant) is big and heavy. (The regular fire extinguisher is corrosive -- meaning once you spray the car, say goodbye to it.)

So at the end, I figure you should make sure you have good insurance in case of a car fire.