If you steal or hijack a fire truck you can press Num + or Caps Lock (R3 for the PS2 and X for the X-Box) to activate bonus Firefighter missions. It allows you to put out cars that are on fire. You will get a certain amount of time to put out the fire or the car will explode. While doing these missions you might find some of the vehicles for the Import/Export garage which can be hard to find otherwise.

This is a great way to get to know the streets which will help you when you do missions or need to get away from the cops. If you put out 60 cars, 20 in portland, 20 on Staunton Island and 20 on Shoreside Vale, you will unlock the flame-thrower at your hideout.

You can find parked fire tucks at the Fire Stations on the different islands. They are marked on the locations map with this icon: