Okay, I was able to find a Windows install with the issue. Turns out that my fix works after all, and I've fixed this for both alpha.twizzle.net and experiments.cubing.net/cubing.js
For anyone curious, it seems that Firefox on Windows easily reports that the mouse has moved a few pixels but that exactly 0 time has passed since the mouse was at its previous position. We calculate how fast the mouse was moving, and since the mouse moved in exactly 0 time we get a speed of
Infinity. This causes further issues similar to
NaN ("not a number") like
@xyzzy suggested. The workaround is to ignore such mouse movements when it comes to calculating speed.