Wow, that's a great question. When I was in graduate school, I got to light controlled fires and put out controlled and wild fires. I think fire ecology is really interesting, but this question had not occurred to me. Here's what I think. Fires need 3 things: fuel,oxygen, and heat. Water can influence all three. If there's enough water, it can smother the fire, by keeping away oxygen. Usually,though, it absorbs heat.

When something like wood is burning, heat causes some of the solid wood to turn into gases, and that is what actually burns. Burning the gases makes more heat, which causes more of the wood to go into the gas state,and so on. That's why fires grow exponentially. So if there's not much heat, then you can have tons of logs but not actually have usable fuel.

Here's where water comes in. Water has a lot of "thermal inertia."This means that it takes a lot of energy to raise the temperature of water. It will also hold that heat for a long time. That's why it's warmer by the ocean or lake in the winter and colder by the water in the summer.

Now, when we put water on the fire, it basically absorbs the heat so that the fuel doesn't turn into burnable gas. Fire burns at thousands of degrees (depending on the fuel). Water only gets to 100 degrees C before it boils. So there's not much relative difference between cold and hot water temperature compared to the huge difference between the fire temperature and the water temperature.

So there might be a very small decrease in the fire-fighting power of warmer water, but I don't think that it would have a very big effect.

On the other hand, air temperature and humidity (the amount of water vapor in the air) have a powerful effect on fires. When it was too hot and dry, we wouldn't set the controlled fires because it was too risky.The potential fuel was already closer to vaporization and there was very little water to buffer the increased temperature of the fire. When it was too cold and humid, it was hard to make things burn.

One way to increase the fire-fighting power of water is to turn it into foam. Why do you think this works?