fire extinguishers, first aid kits, and all this n that n whatnot

So, I been makin hay for these in the missions... where are they; how do I make use of them; are nutritous snacks included?

In your garage, go to service and add them to your tank along the bottom.
Careful though, some bigger expensive ones cost gold each time you use them.
In the game, press 4,5 or 6 depending on where you put them and if your track is broken or you're on fire, etc.
They fix the track only once and an extinguisher doesn't always get the fire out.

"Large" kits cost gold, but also have ambient aid to your tank, e.g. The large fire extinguisher gives additional fire protection before it is used, so you are less likely to catch fire in the first place. Similarly, the large repair kit speeds repairs and the large first aid kit protects your crew from injury.

I highly recommend he large kits if you are going to tank even a little seriously. They were on sale when I got them from the "gold" site: 50 kits for ~$6.

Small kits cost silver but are only single use and do not provide additional aid.