Grand Theft Auto 3

Fire Truck missions are side missions that involve you driving around the city putting out burning cars with the use of the mounted fire hose on the fire truck. In order to start these mission you must first steal a Fire Truck. In Portland you can always find one at the Fire station which is located Harwood area, near the junkyard. Once inside the Fire Truck, press  to start the missions. 

Once they have started, drive to the blip displayed on the minimap and use  and the  to direct the water hose at the fire. Once the fire has been extinguished, the next location will appear on the minimap. The Fire Truck is a very difficult vehicle to manoeuvre and control. I highly recommend you use the handbrake to slow down as opposed to reverse, as this will result in less sliding. During the Fire Truck missions, the number of fires you have extinguished is displayed in the top right of the screen. Each fire you put out will reward you with a small time bonus. Once you reach 20 fires within a single mission the trophy will unlock. If at any point you exit the Fire Truck, run out of time, or you blow up the Fire Truck, the mission will fail and you will have to restart.

The below video shows the location of the Fire Truck in Portland and how to unlock the trophy.