You can use the flamethrower to create a field of fire in front of you which harms players and ignites them.
To use the flamethrower you need golden shovels which are the ammo. They can be bought for 80 Dollar in the fire department too. If you have those items you can simply activate the flamethrower with a rightclick. It will be turned on until you rightclick again, put it away or you run out of ammo.
The flamethrower can be bought in the fire department for 250 dollars.