Out of curiosity, What flammable gas will be most likely used by the FIJI?
There are many gasses with a density lighter than air, but few are flammable.
More common combustable gasses include Hydrogen (H2), Methane (CH4) and Ammonia (NH4). Since Methane is readily produced through the digestive process of many creatures (microbial respiration), it will be one of the easiest gasses for the FIJI to form and collect in it's canopy/whatever the organ it called that holds the gas. (I like the name "float-bladder").
Methane burns cleanly, which means that out of all fossil fuels releases the lease carbon dioxide when burnt, making it a "cleaner" alternative to coal or LPG. When ignited, FIJI's are going to burn at least 75% of their methane store immediately to form carbon-dioxide and water, meaning there is going to be little climate destruction caused because of them in this regard.
The Canopy or "bell" of a normal jellyfish is made from a non-living material called "Mesoglea"; primarily made from water, it gives the fish it's typical jelly-like appearance, also making it the definition of non-flammable (hence, being the bulk of the FIJI, this means that when the creature explodes the organic debris that drops to the earth will not be as ablaze as one might first expect). But the FIJI is not a "normal" jelly fish, so it would be naive to assume it would retain the same gelatinous water-bell; instead we must assume it has evolved an even lighter, more elastic, parachute like material to use instead, a material that can not only hold methane but one that can burn long after the initial spontaneous combustion has occurred, almost like the wick of a candle.
With such a canopy, the almost harmless explosion process takes a more dangerous turn.
After the initial explosive event, the FIJI's burning remains will rain down spectacularly from the sky. As this combustion process takes place at an altitude of +3km, the embers will have a long way to fall, and at the mercy of the wind will be carried in all directions - hence possessing a very large potential burn radius. They will land in random locations where if in close proximity to fuels in the form of grasses, timber or flammable liquids ( and provided that it is still alight) will quickly start spot fires that have to potential to then grow into large untamed blazes, posing great risk to life in nearby areas. With little control over when explosions will take place and where exactly fragments of the FIJI will fall, there cannot be much done to prevent these fires flaring up - besides eliminating all dry foliage or the FIJI population in general.
NOTE: The probability of spot-fires burning out of control will more or less depend on the weather conditions as much as the burn-time of the FIJI itself, which will be dictated by the material it is composed of. Depending on these factors, the FIJI can either be a relatively harmless creature, or a devastating force of the natural world!
ADDITIONALLY: Given that these creatures travel in large groups, the combustion of one may cause those nearby to ignite also, leading to the chain explosion of dozens of FIJI and potentially catastrophic damage to the surrounding ecosystem (huge amounts of firry debris from said event more than capable of causing severe bush-fires or the destruction of countless homes in an inhabited region). The population of your world will need to have preventative measures in place, like water-bores and other water catchment/distribution systems, in case one of these fire-storms takes place nearby, or they'll be in a lot of strife. FIJI therefore could also prove an interesting dynamic in war, an enemy "blowing" a group of creatures into opposition territory and exploding them one way or another.