You don't need to fire the flash, only power it on. The short-term energy storage in a flash is the flash capacitor. It has a very high energy density and very high discharge currents. That does not leave a whole lot of leeway for sacrificing performance for durability. The energy in a capacitor is stored in an electric field between two electrodes. The shorter the distance between electrodes, the more energy can be stored. And the more likely there is going to be a breakthrough between electrodes that, at the involved energies, will burn a hole ruining the capacitor.
The electrodes of a capacitor are separated by a dielectricum. Now in an electrolytic capacitor, one electrode is a wrapped metallic foil, the opposite electrode is a liquid (typically drenching some paper-like stuff) and the dielectricum is not explicitly constructed into the capacitor. It is a thin oxidised layer on the metallic foil that is formed by applying and very slowly raising voltage. As the layer grows, the voltage it can withstand grows.
A capacitor that is in use and under its nominal voltage, "self-maintains" its isolation. Not using it causes the layers to deteriorate. The first consequence is leakage, using more battery power while the flash is idle than usual. But if the capacitor has been unpowered for too long, you may get strike-through, burning hole in the foil. Some capacitors are "self-healing" where they only lose capacity but the hole does not result in other consequences. But flash capacitors tend not to have the reserves for this kind of behavior. Once they strike through, you usually have them strike through at the same point again and again.
You have the same effect with old unused electronic music instruments: powering them on may cause their capacitors to explode. Most power supply capacitors in newer years are less critical in that respect.
But flash capacitors don't have a lot of leeway and operate at really high voltages.
Firing the flash is not important. Charging it is. The worst you can do is switch it on and leave it on when the capacitor is already endangered. You start by switching it on very short (so that it cannot reach full voltage) then let the voltage decline again. Do that for a few days before keeping it powered up for more than a minute. After a few weeks of that, keeping it on for longer times may be dared.
And yes, I have several fatally unused flash capacitors in my gear history.