Of course, this only works if you spot the small fire quickly. By the time smoke is high up in the air, the fire has likely grown larger than this drone can handle.
Also, some ignitions come from known sources (e.g. powerline downing), and monitoring and localization already exists in some cases (PG&E).
That said, what would be really useful is a surveillance drone we can send out when someone phones in a smoke report. 9 out of 10 times the call is nothing, but we still have to suit up, drive to the station, get in the engine, and go investigate.
Water is heavy and it takes a lot of it to counteract the heat generated by a wildland fire. Which is why water is not the primary tool for fighting wildland fires: McLeods, chainsaws, and bulldozers are. Fuel removal along a one-dimensional perimeter is much easier than bringing in massive quantities of water across a huge two-dimensional area.