You can pump as many talents and tweaks into making Firestorm a more compelling choice, but the spell and the builds around it will never succeed in a realistic environment so long as a boss or clump of mobs move the tiniest bit off the red circle.
Another user suggested making firestorm act akin to sPriest’s entropic rift and personally I’d love to see that, at least then firestorm would be worthwhile to pick outside of super stationary fights with a constant 3+ targets like VOTI’s council fight. That’s legit the only time I’ve seen firestorm considered.