FF takes longer just because the script is pretty broken, imo: it only processes 4-5 chars per 16ms frame, and reflowing/painting each frame tick here is the expensive thing in all browsers. As a result, the behavior of the script in wall-clock time is O(N^2/K) where N is the number of chars and K is the number it processes in a single tick. Furthermore, its CPU consumption is pretty much constant no matter what K is for a wide range of K.
