Firestorms are wind storms where the movement of air is sustained by the heat of a large fire, rather than solar energy from the sun. They are fairly common in nature, and we have managed to create man made versions as well.
The smaller version might be the fire tornado.
|Fire Tornado (thumbnail - original here)|
This detail from a firestorm at Yellowstone in 1988 (here) I think helps indicate the difference. The fire tornado is a discreet element, and the firestorm is more general body of fire spread out over the earths surface.
Firestorms can cause a large number of casualties. Historically many of the great conflagrations that burned down whole cities likely had a firestorm element to them. The allies during World War 2 figured out with the right combination of high explosives (to disrupt fire services) and incendiary (early napalm) you could start massive firestorms in German and Japanese cities. The firestorm raids on Tokyo caused more casualties than either of the atomic bomb drops.
Cities used to burn down to the ground occasionally. Two elements are likely the most responsible for reducing this risk. The most obvious is modern fire services, sometimes combined with modern sprinkler systems that contain the fire so that the fire services can arrive in time. The second is the limiting of combustibles in construction on larger buildings. Note, in somewhat of a retrograde, modern fire codes tend to allow for relaxed limits if fire protection sprinklers are present in a building. Finally, set back rules, keep smaller residential buildings further apart. That this is an important element can be seen from the areas we do get large scale multi-family fires: in townhouses.
In our typical apocalyptic, societal collapse, how many of these elements are present? One. The setback distances. Granted there is a hit and miss nature to these conflagrations, but the idea that rioting, or gang activity are going to be the major violent threats might need to be reconsidered.