Is Firestarter a % chance to proc per target or per spell cast?

[yurpcraft]

Against 5 targets, putting 1 point into Firestarter and casting dragon's breath or blast wave nearly guarantees a proc.

https://classic.warcraftlogs.com/reports/mBr79ghfdQaTwJ6t/#boss=-3&difficulty=0&type=damage-done&source=1