The firefighter isn't particularly useful at the moment, this suggestion is meant to change that.
This new job would be similar to the terrorist job, in the sense that one would need to level it up and buy materials in order to set buildings on fire. Lighter fluid, to light the buildings on fire, could be sold at the bomb shop, for the same price (or lower) as C4, by gallons or liters.
Anyway, this pyromaniac would use lighter fluid to light buildings which are not blowable on fire, like 24/7, LAPD, gas stations, and so on.
Firefighters would show up to speed up the progress of extinguishing the flames, otherwise, the flames would extinguish in ten minutes or so, but with firefighters, they would extinguish in one or two minutes.
Hell, he could even set himself on fire, cars on fire, set an area on fire, possibilities are endless!
Feel free to add to the idea or let me know why this is a ridiculous idea.