What are the odds of a red dragon's breath setting the forest on fire?
Given that, what sort of fire break would a frost spell create? An area effect one, of course.
If the description speaks only of the damage at the point that it's cast, it would have to be timed quickly, but heat is one of the three things that a fire must have to burn. (And, of course, if you do stop the fire with it, the burned out forest is the fire break -- for that stretch.)
Or do you have to cast it directly on the fire? (Which would have the advantage of allowing targets as well as areas.) And how many hit points does a fire have? Not a fire elemental -- a fire.
