BLM: Endgame guide and rotations

[Pacifyer]

Didn't read all the posts. I don't like using my firestarter during my fire phase because it actually slows down my next fire spell. I save it till the end of my fire phase and hit it before my blizzard 3. Or if my last fire spell qued it up I can use it after all my mp is back. Is the slower casts really worthit?

This is why I did write guide Perfect example of what you SHOULD NOT do ever.
Firestarter proc does not slow your next spell by any means. What you consider "slow" is the entity called "GCD" or Global Cooldown. It is a cooldown triggered by all spells and it is primarily intended to get your instacast spells under control. Basic value of global cooldown is 2.5 seconds which generally means that you can not cast spells more often than every 2.5 seconds. If you cast a "hardcast" spell like Fire1 you probably won't notice this GCD as it will end up right when you end casting it. But if you use any instacast spell like Scathe, Firestarter Fire III, Thundercloud Thunder, or Swiftcast some spell - you will see this GCD in effect. You can also see GCD if you use some spell which has cast time less than 2.5 seconds (like Esuna on WHM) and you may consider a delay after the cast as a server lag or something alike.
So Firestarter makes your Fire III instacast and cost you no mana. This will effectively make it's cast time equal 2.5 seconds (against 3.5 without proc), cost you no mana (which is the main issue during fire phase) and do a ton of a damage as Fire III is the most hard hitting nuke you have (don't count Flare). So by no means there is a reason to hold your Firestarter procs. They are a direct DPS gain and should be used ASAP.

P.S. However there are exceptions - several spells which do not trigger GCD. They are very specific and are mostly intended to use in conjunction with other ones and have a cooldown of their own. Those are Swiftcast, Transpose, Convert etc.