Firestorms are not easily created at will, they're critically dependent on the physical conditions of the fuels and weather. Just look at WWII firebombing campaigns, for instance.
In mid 1943, Hamburg's incendiary bombing with napalm, white phosphorus, thermite, magnesium, and high explosives was successful due to the right mix of environmental conditions that allowed it grow to apocalyptic proportions. The code name Gomorrah turned out to be an appropriate moniker for this military operation.
The next cities where firebombing created similar firestorms and destruction were Dresden and Tokyo in 1945. The absence of similar firestorms in the intervening 2 years wasn't for a lack of trying, it's just not that easy to pull off a catastrophic firestorm, even when the "arsonists" have massive resources to throw at "the problem" and unhampered access by air.