I mean the a Fast fire 3 doesn't take much more time then an instant fire 3. You have the global 2.5 sec CD, and the only benefit would be you get an Astral 1 fire 3. I'd rather take the astral 3 fire 3. You can jump to cancel Blizz 3 fast enough in my experience and Blizz 3 casts really fast anyways so you don't lose that much time.
Depending on your latency I guess.
I would and do cast Blizzard I in that case. Who knows, you might get another thunder proc during your fire cycle.
