I got fire starter literally yesterday and had absolutely no problem with it. Took me about 3 minutes to figure out the delay and then another 2 to realise it can actually be helpful. As other people have said you obviously lose DPS if you cancel an attack and if this happens to fall on the last of your mana you can Bliz 3, bliz 1 or thunder 2 then Fire 3 to immediately get back into kicking arse.
Also if i'm just fighting trash mobs, feeling lazy and actively want to avoid this happening I just wait until I see the Firestarter notification pop up on my char and then hit my Fire 3.