I can answer the firefighting question... there are three required elements for fire: heat, oxygen, and fuel. Take out any one of the three members of the "fire triangle", and you end the fire. All firefighting agents work to remove one of those three elements.*

Water works by removing the heat element, because water absorbs heat and evaporates, which is a highly endothermic process. It cools a fire the same way sweat cools your skin.

* There are some uncommon agents that are exceptions to this rule. They remove neither heat, oxygen, nor fuel. They're said to operate on the free radicals that allow combustion to continue as a chain reaction. In some fire studies this is considered the 4th side in the "fire tetrahedron," and in others it's the three points that connect the sides of the fire triangle. Either way, this is pretty advanced chemistry stuff here, so feel free to disregard.