In CERT Training, participants are taught to not go after anything larger than a trash can as a rule of thumb.

(Note: The "rule of thumb" is also used to determine if you're far enough from danger for most things: hold your arm at arms length, close one eye, and look at whatever danger there is with your open eye and put your thumb over it. If it's completely blocked by your thumb, you're probably far enough away. Yeah, doesn't work for everything...).

Ensure that your fire extinguisher is mounted, and if you cannot mount it, make sure its in a heavy bag (to protect the pin and handle ) and that the bag cannot fly around in an accident.

You may be able to purchase an OEM extinguisher mount for some cars, as some countries require rental cars (maybe all?) to be equipped with a fire extinguisher and first aid kits.