I like that someone is thinking of better ways to fight fires. However, I have serious doubts about this idea. The about of heat and turbulence generated by a big fire is mind boggling. Airships try to avoid turbulence because they don't handle it well. In addition, the hot air above a fire is less dense, reducing the airship's lifting capacity a great deal.

The firefighting planes go in low for a mundane reason - if they drop from too high, much of the drop will evaporate before it hits the ground. It's similar to the weather condition known as virga where rain falls from clouds but evaporates before reaching the ground. I see that all the time during hot, dry conditions.