I never save my firestarter procs, but I do occasionally have one left over if it procs during my Blizzard III cast since I don't wait to see if the last Fire procs or not. When that happens I never start the next damage rotation with the Fire III proc, but use a Fire first to take advantage of the fast cast time which also erases the Umbral Ice effect eliminating the reduced damage on the proc. It's both faster and more damage to do it that way. I haven't tried using transpose to put myself into a single stack of Astral Fire and then leading with the proc, however, and that seems like it would be effective even if not using Scathe as the filler. I'll have to test it.