It could be that his logic optimizes for chrome. I'm sure with a bit more time, he could make it work smoothly in FF... Or fuck it, just write it in ASM, which would be insanely faster.
The issue isn't any sort of optimization for chrome, FF just chokes when it tries to render SVGs quickly. See also: http://roadtolarissa.com/twisters/
I doubt the issue is the JS logic. Firefox on Mac has nothing like the lag people are talking about on Linux/Windows, which suggests the issue is somewhere in platform-specific drawing code. Hard to say for sure without a profile.