Actually, I did think about this.
The window hammer/seat-belt cutter goes neatly & quite firmly in both the front end of the clicked-shut door bin or in the hidey-hole with a drop-down lid near your right knee, from either of which I don't think it would jump out to do damage. (That said, in a big accident, anything's possible, of course.)
(I wouldn't, however, put it in the flimsy, limp-wristed cradle it comes with.)
The fire extinguisher's in a padded bag in the boot, together with other items, like a plastic bottle of engine coolant, light, Tyreweld, Swiss Army knife ...