As a fireman you can extinguish fires from a great distance. Not every fire is that easy to reach, though, so you'll have to aim your fire-hose very well, and delete obstacles. The fire will only go out when you've hosed a certain amount of water onto a little house. Use your water sparingly, because once there's nothing left anymore, you can't fill up your supply and you'll have to start extinguishing all over again!
Use the mouse to aim and click to hose water. If you hold the left mouse button, the water will keep on streaming.