[WayneS]

Forest fires tend to be a lot larger than swimming pools.

I think the main reason this method is not used to fight forest fires is the astronomical cost of generating and transporting a sufficient quantity of liquid nitrogen to effectively fight a large fire.

[batterycommander]

Nitrogen can be compressed, and even liquidified, solving the volume problem, and even in uncompressing the gas will cool the fire.

[HiTech RedNeck]

Also, the area would need to be cleared of personnel, who would otherwise be smothered too (along with most of the fauna).