The problem with having a fire extinguisher in the boot is that most accidents here are rear end. If you get hit hard enough, you won’t be able to open the boot to access it.
I have needed one in the car once and I was very glad to have it. Used them in the garage a few times too.
Rally cars usually have 3 extinguishers. One hand held and 2 plumbed with pipes and nozzles. One plumbed for engine fires, pull the lever and it empties in the engine bay, the other one sprays on the driver and co drivers legs.