Effective yes but how cost effective was it compared to other systems. Getting a firestorm going by the way often took a mix of HE and incendiary and perhaps even FRAG bombs. How many did you have to drop and how much did they cost? How many aircraft and crews did you loose in the process? One of the things that made the Atomic bomb so effective is that it only took one plane to take out a city. I seem to recall the Dresden raid as being at least 3 and perhaps 4 seperate waves with hundreds of planes. Not sure about Hamburg. Nor was it possible to tell when you would get a firestorm, they were actually quite rare.