When I live in Belgium circa 1995 a hand held fire extinguisher mounted inside the passenger compartment was legally mandatory. When I brought the car I had, back to France, before European homologation standards, I was told to remove it from the inside and mount it in the boot/trunk if I wanted to keep it, as a five kilogramme hunk of metal breaking loose inside the car in an accident is a dangerous projectile. I agree, so trunk mounting for a hand held bracket mounted extinguisher.
Depending on the source and size of the fire the treatment is different. For an electrical smouldering fire, isolate the battery and use a dry powder extinguisher. Dry powder is effective against electric smouldering fires. I and my son saved his BMW this way. The fire was due to a short.
If the fire is near fuel lines or the tank, I agree with the comment of getting everyone out, stand well back, call the professional emergency services and watch it burn. Have good fire insurance. Even in an accident where there is no immediate fire, get everyone out and away, unless serious injuries are suspected, in which case do not move the person unless fire actually breaks out. Wait for the professionals. The fire may break out later, as in the Greek car accident mentioned in another thread.
Fire extinguishers have "sell-by" dates and they do deteriorate with time. When I worked in the petroleum industry we were trained to deal with small petroleum fires in laboratories. Even a few litres of gasoline, goes "WHOOF!" when lit and is very hot, very rapidly and is intimidating. As they say in children's programmes on the television, this is not something you should try at home! (I never did the more impressive refinery fire training!!!)
If you must treat the fire, while an injured person is extracted for example, aim the jet of the extinguisher at the base of the flames, not just in the general direction, and get down low, it is slightly cooler and the flames rise off the actual point of combustion.
One 5Kg extinguisher does not go far.
If possible protect yourself with clothing, gloves in particular. Bare skin barbecues rapidly.
Hope this is advice nobody ever needs!
'97 Rosso Corsa 550 Maranello