Damage / Buff calculations based on animation is problematic.

[Altena]

I have ever missed a fire starter proc. not sure some people freak out over this. I just finish the cast and then use fire 3. It was worked every time and never wasted a proc. the proc does have a time limit though and if you don't notice it maybe that is how you are missing the proc.

Firestarter can proc on the next Fire (and happens often) - ie you are wasting a proc.

For example:
Cast Fire
While starting next Fire, Firestarter procs.
Finish casting Fire and Firestarter procs on this one too..

ie you waste the first proc if you decide to finish the 2nd Fire cast.