The drones you're thinking of here (FPV stunt drones, racing drones) have the most demanding discharge profile of any use for lithium batteries. The whole dron

The drones you're thinking of here (FPV stunt drones, racing drones) have the most demanding discharge profile of any use for lithium batteries. The whole drone is built around draining current from the smallest possible battery, as fast as possible, right up to the limit of safety. Charging is also a faster-is-better thing, from a pilot's perspective. This kind of maximum discharge raises the risk of dendrite shorts considerably, which are what pops a lithium battery usually. So a firebox is a good idea for this case, it's just inherently dangerous to be right at the limit of energy-per-gram and discharge capacity all the time. The chemistry is also never LiFePo, which is too heavy.