The proc happens when the damage lands, that's why it takes a second to see it proc.

Usually at that level, you just keep spamming fire. Sometimes if you have a thundercloud proc you can throw that out in between fires to see if the firestarter proc has happened. If not, you just keep going. Firestarter procs are good damage burst when they happen, but don't go looking for them, use them when they show up! And if you get one on your last fire before going back into ice, save it. Wait til your mana is back up then transpose back into fire and throw it out to start your fire rotation again.