The fire truck is found at the Southeast corner of the map, where road meets Beach. It can spray a harmless stream of water. You can do a firefighter job when you get inside this vehicle.
You can do a little job while you get inside this red truck. There are 10 levels, and your job is to clear the fire from the house and street using the sprinkler in the truck. After you finish all the 10 levels, you will got $1000 and FIREFIGHTER achievement.