Ideally this would be some sort of sun or star-related natural disaster that would affect the entire planet and make life outside of protected/shielded areas extremely dangerous. This is a human-inhabited planet, and while many people will be living in domes built specifically to guard against this catastrophe, others will be left out in the open to fend for themselves and while I don't want them to be wiped out completely, their chances of surviving should be slim.
According to Gizmodo, a coronal mass ejection (CME) isn't going to do more than fry the power grid and anyone/thing using it at the time. CMEs aren't bursts of fire. Instead, they're superheated charged particles. When the CME reaches the Earth, they interact to produce geomagnetic storms and spectacular auroras, but no fire. So they're pretty, but not particularly dangerous to those protected by the Earth.
Gamma ray bursts (GRBs), on the other hand, are the most energetic explosions in the universe and are immensely dangerous. If the Earth were hit by the one of these, there would be a worldwide extinction-level event and the atmosphere would be ruined for decades, centuries, or longer. Anything on the surface directly in the beam's path would likely be incinerated in short order. You would need a shelter in the deep ocean to have a decent chance to survive such a catastrophe. Of course, there's some evidence that mankind has endured such an event, so maybe not so farfetched after all...
Would either of these cause firestorms? A CME may spark fires indirectly, by causing a geomagnetic storm that unleashes a vast amount of lightning that starts fires; this is how an electromagnetic pulse can conceivably start a fire, and the two operate on similar principles. A GRB, on the other hand, can definitely start fires. Lots of fires.
It's unlikely that an extinction-level GRB would cause a firestorm. Not impossible of course. Put it close enough and you can have it vaporize the biosphere ... and "close enough" may mean a few thousand lightyears with its axis pointed our way. These are events which make supernovae look small!
But the orders of magnitude less unlikely scenario delivers a pulse of radiation that doesn't even give us lethal radiation sickness. What it does do, is ionise a significant fracation of the O2 and N2 in the atmosphere. These recombine to Nitrogen oxides. This in turn destroys the ozone layer in seconds on one side of the planet and in days on the other. The Sun's UV does the rest. Afterwards, if you survive the sunburn, there is decades of nitric acid rain to look forwards to. Exit most of the terrestrial food chain.
Terrestrial life would be decimated or utterly wiped out depending on the GRB strength. Marine life would also suffer badly by acidification of surface waters. Deep water life might not notice much. There is a lot more mass of ocean than air.
If you want a firestorm you should probably invoke a large meteor like the one which did for the dinosaurs or a little larger.
A sufficiently large CME? You bet it could cause firestorms or worse!
Coronal mass ejections subject the earth to increased radiation, much of it quite damaging to life forms, if not shielded by the magnetic field.
So you don't want to be at the poles during a bad solar flare.
A sufficiently energetic CME/Flare could overpower the earth's magnetic field -- a really big one could cause intense heating (even firestorms) by hitting the atmosphere and surface with high-energy protons and photons (UV, x-rays.)
FYI, I recommend Larry Niven's short story, "Inconstant Moon." https://en.wikipedia.org/wiki/Inconstant_Moon (which was also made into an Outer Limits episode.) Enjoy!