Let's take this image as our fireball in question:
And let's further assume that it travels about 30 meters before burning out (twice the range of a WWII flamethrower) and travels at the speed of 20 m/s (which, apparently, is about how fast a 10-year-old can pitch a baseball). The flame's temperature is in the region of 1,300 celsius, in line with actual flamethrowers.
What how much energy, measured by kilocalories, would be required to fuel such?