One additonal note, since Fire Blast is an Instant Spell and is also available to be buffed with a talent point, I have been mixing it into my rotations on the 2nd and 3rd phase of a fight. It has been proc'ing AM alot. When I do throw it in, being keybound, I hit that then Barrage almost instantly if I don't see the proc from Missiles. That gives me 2 spell chances before going back to AB again.
I believe using this will work out better than mixing in Flame Orb.
Fire Blast also costs 3.8k mana, and doesn't benefit from arcane specialization. Consider that casting 2 Arcane Blasts is still cheaper than a single Fire Blast.