It literally is not a problem once you hit 60 and get Fire 4 as you should be saving the Fire 3 proc for movement or you can slap it onto the end of your second wave of Fire 4s so that it doesn't go to waste.

As for the proc itself, it applies the buff about half a second before the end of the cast, so you have time to react. You might not get it everytime, but it isn't crucial at the lower levels.