A firestorm is a conflagration which attains such intensity that it creates and sustains its own wind system.
A firestorm is created as a result of the "chimney effect" as the heat of the original fire draws in more and more of the surrounding air.
This draft can be quickly increased if a low level jet stream exists over or near the fire, or when an atmospheric temperature inversion cap is pierced by it.
As the updraft mushrooms, strong gusty winds develop around the fire, directed inward.
This would seem to prevent the firestorm from spreading on the wind, but for the fact that tremendous turbulence is also created by the strong updraft which causes the strong surface inflow winds to change direction erratically.
This wind shear is capable of producing small tornadoes or dust devils which can also dart around erratically, damage or destroy houses and buildings, and quickly spread the fire to areas outside the central area of the fire.