That Poly Performance mount looks pretty neat... a little pricy though (considering multiple vehicles). I saw a thread in a 4x4 forum once....where a guy took an adjustable "water bottle" cage/holder made for bicycles that he bought at Walmart and the smaller sized fire extinguishers fit perfectly. I believe it had a rubber strap on it, the mounts they come with are pretty flimsy.
Well, if you want to save some cash and make something yourself, here's a write-up on a home-made version of the Poly Performance quick-release mount using $20 of parts:
http://www.jkowners.com/forum/showthread.php?t=31906 A long time ago in a far away land.... "somebody" forgot to put the oil fill cap back on an Toyota SR22 engine I was driving. I pulled into a Checker Auto Parts to inspect what/where/why there was smoke starting to come from under the hood.... opened the hood and ...whoosh..... flames. I dropped the hood down (not allowing it to lock) ran into the store and came out within a minute with a large fire extinguisher and it went out quickly. I don't know why anybody would not try to put it out.... fight or flight I guess.
Everything was fine and I went on my way... I now carry a fire extinguisher in every vehicle. At the very least to help anyone else out. When I took my C.E.R.T class I remember the Fire Captain showed us how to work the nozzle in a back and forth (left to rightish) or circular motion and it was very effective. Much more effective at putting a large fire down than just aiming for the center. For what it's worth;)
Generally speaking, if there's an engine fire you're going to need to replace all the wiring, hoses and belts. Things like the starter, alternator and AC compressor may/may not fail immediately, and so might be overlooked by a repair shop/insurance adjuster. On top of all that, there is the risk of personal injury involved in being close enough to a burning vehicle to try to put out the fire. I can't fault those who would rather stand back and wait for a professional to address the situation. After all, it's just a vehicle.
And yes, you do have to play the fire extinguisher around the fire, not just dump it in the middle. Aim at the front of the base of the fire and spray, work side to side, evaluate the effect and spray additional hot spots.