The fire extinguisher will put out a single square of fire, but is not effective against slime. You'll have to get pretty close to the fire though, because this puppy only has a range of 1!
This is now longer a toy anymore. The Super Soaker 40000 allows you to extinguish fires, and wash away slime at a great range. It can even be used as a weapon against the smaller foes.
Bombs away! If you want the fire out quickly, this is your best bet. With an area effect covering 9 (count em, NINE) squares, you should be able to put a kink in the local pyros plans in short order.