40k dead; almost all of Hamburg destroyed
1,500 ft tall; 1,500 degrees fahrenheit; 150 mph winds
Hamburg, Germany; 53.33N, 10.00E
July 27th, 1943
On of the most common ways to create a firestorm (opposed to happening naturally) is bombing, like during WWII in Dresden and Hamburg, the latter being more intense. Because it's not possible to separate damage and deaths done from the bombs vs the firestorm, they will be combined.
In July 1943, Allied forces staged a bombing of strategic areas of Hamburg. It was apparently really nice and warm, and had been for some time, so there wasn't alot of environmental "noise" to affect the bombs nor the resulting firestorm, but enough of the history lesson. Bombs create explosions create fire create drafts create oxygen (read fuel) create fire. Rinse and repeat those last two parts until all physical fuel has been eaten. 40k+ civilians dead and almost an entire city destroyed by the firestorm. The manifestation of fire in question was: 1500 ft tall packing 150 mph winds @ 1500 degrees, enough to melt streets and kill people from suffucation (eating O2) rather than incinerating them. Let's put this in perspective...
A quick web search tell me water tower are around 200 ft tall. So, 7 water towers tall, winds of an EF3 tornado (middle strength), and hot enough to ignite bulidings without actually touching them. People were being sucked into the firestorm. Water molecules were probably being split into separate oxygen and hydrogen, creating more fuel. This is one of the forces of nature that humans cannot combat. Just get out of the way ASAP. Here's a video of this beasts (very) little brother: