Just tested on a target dummy, if you're casting Thunder III during regen phase ( you should be casting Thunder during regen) you sit on full mana for nearly an entire tick, that's less Fire up-time and less Firestarter up-time. Even without damage meters you would have a hard time convincing me that 30 potency on cast and 100 potency at a 5% chance per tick of Thunder is worth a single Firestarter proc.