Whatever you choose to do with the fire extinguisher be aware of these very important issues:

1) in an accident anything that is loose or could come loose will be a missile aimed directly at your head since Murphy will ensure that you get hit in that direction right after installing the dangerous object. Don't mount that fire-extinguisher at or above waist level!

2) Any non-crushable item (fire-extinguisher, glass bottle, brick etc that can, will get under the brake pedal as soon as you put it in the car. Don't mount that fire-extinguisher on the left hand side of the front floor

3) anything you can't reach and grab when you need it in an emergency, you don't have in an emergency. The trunk is not convenient for anything you might need on a truely urgent basis.

I keep mine loose on the front passenger floor. Meant to mount it but never got around to it. The plastic mounting bracket doesn't look strong enough to matter in a colision but would keep it out of the way better during regular driving. OTOH, I wouldn't want a better bracket since I don't want to have to fight too hard to get it out of the bracket when I need it.