From the pretty extensive testing I've done (multiple 20-30 min tests), interrupting for Firestarters was DPS neutral, at best. And that was back before the buffs to Fire 1 and Fire 3 (of which, Fire 1 received the stronger buff, %-wise). So, the case for it now is slightly weaker than it was.

The only time you should interrupt for Firestarters is to fit more damage into a short period of time (bursting adds, certain phase pushes).