This is my rotation for BLM so far:

Blizzard 3 -> Thunder 3 -> Fire 1 -> Firestarter -> Swiftcast -> Fire 3 -> Blizzard 3

Blizzard 3 ( When 500 mp )

I throw in the 20% Damage buff when I start casting Fire and gain 3 stacks of Astral fire. When Firestarter procs, I like to cast swiftcast and follow it up with another Fire 3 just for the bonus burst before switching to Umbral Ice 3. Throw in TH whenever.

I dont instantly use Fire 3 after using Blizzard 3 because I wont spend as much mana right away unlike with with Astral Fire 3 when casting Fire 1. In the end, this allows more Fire 1 casts and best of all more chances for Firestarter to proc.

Also another good tip is to throw in this after that full combo up there:

Firestarter -> Swiftcast -> Fire 3 -> Blizzard 3 -> Scathe

Tell me what you think