Wat.

Firestarter procs are going to be one of your three hardest hitting skills, among flare (which should be used sparingly since it drains all the mana you have remaining. On boss fights, I only use flare when I have swiftcast and convert available) and thundercloud procs. You want to use these firestarter procs when you have 3 stacks of Astral fire whenever possible, not after you've already switched to Umbral Ice. More damage with your fire stacks and no mana cost, so... Use up your firestarters before you start to regen mana.

Starting your rotation should look something like: thunder 3 > fire 3 > raging strikes > fire until low on mana, throwing in procs wherever possible > swiftcast flare > convert > fire > blizzard 3 > thunder 2 > fire 3 > fire until low on mana > blizzard 3 > thunder 2, etc. You'll want to use thunder 3 whenever you proc a thundercloud, as well.

I've seen too many BLMs at end game not have a clue how fire 3 and blizzard 3 work, it's painful to watch.