I'm playing fire on Fallen Avatar.
Can someone explain to me why "firestarter" is such a popular talent on warcraftlogs. It seems very lucklster to me unless i'm missing something.
Grats, you did shittons of damage for the first 15 sec of the fight and for the first "attack maiden phase".
I used to be on the same boat as you, and could never understand why people used Firestarter until I tried it out myself.
Think of Firestarter as essentially a tad weaker Combustion. Every Fireball and every Pyroblast will crit. So your opener and first few seconds of the fight look a bit like this:
Fireball + PF => Pyro + FB => Fireball + Pyro => Fireball + Pyro => Pyro + FB => ...
Essentially you're getting a Pyroblast every other cast, using 1 charge from Phoenix Flames and having 1 charge of Fireblast on CD (it serves us no purpose to not have those CDs replenishing) for extra proccage.
Not to mention, you're likely to get bracer procs that will crit... and if things align well enough for you, you'll get a bracer proc around the 91-92% mark, which you can then start casting, hit combustion at the tail end of the cast and go into a full blown combustion burn.
Assuming the first 10% of the boss took ~15 seconds or so (varies on difficulty and your guild's DPS performance), and our new longer combustion (14seconds at least) and you just had half a minute worth of guaranteed crits on our most potent spells.
Also, I may be wrong here, but, I'm fairly sure Mirror Images also benefit from Firestarter (at least my images tend to have 70ish% crit at the end of the fight -- with my crit rate being 44% on the sheet), and that's a huge boom for your opener as well, seeing as how firestarter+combustion will effectively mean your first MI gets nearly 100% crit rate on their spells.
This opener will put you up there with the burstiest classes and will carry you through until the next combustion burn.
It's also, as you correctly pointed out, a great asset on a fight like Fallen Avatar where you're likely to get two rounds of firestarter, one on Avatar, one on the Maiden.
tl;dr -- Firestarter is something that isn't intuitive to a lot of people, it wasn't for me. But once you play around with it, you'll miss it on the fights where conflag is the better choice.