Originally I was thinking of air dropping frozen blocks of carbon dioxide, then later I was thinking nitrogen.
Air dropping was just the more dramatic end of it, and of course this can also be delivering by land vehicles.
Nitrogen is probably not a doable choice, it freezes at such a low temperature it would be difficult to manufacture.
The reason I found nitrogen as the choice of frozen gas to use is because I thought it had the characteristic of forming a layer near the ground. Of course just until it gets mixed into the open air. But apparently that is not true. And it having little impact on the environment.
Maybe there is another gas suitable for this, but carbon dioxide looks like the choice from what I know.
The concept is by placing long and wide rows of dry ice (or whatever frozen gas) that as they melt, they hug the ground a bit. As a fire approaches and they get warmer they release more gas. The gas encases the vegetation to prevent it from burning.
If this works, it can only be useful at low levels, and not very dense or deep vegetation. And with no or very little wind. So not a major solution, but can maybe help break progress of a fire in some locations allowing fire fighting efforts to concentrate nearby and possibly be more effective.
I expect this has been experimented with, and it was determined to be too impractical and little effect. I'm still hopeful though it might prove useful, if techniques are fully developed.