The only time I cancel for firestarter is when I have already started casting blizzard 3 to replenish mp but the proc hadn't appeared before hand.

The obvious reason being the fire 3 will be under astral fire and if I waited till after blizzard 3 I may have to wait on mp tick to use it.

However I do also sometimes keep them depending on if its a boss because I will usually reapply lightning after blizzard whilst I wait on the mp tick assuming its not instant.