The rendering slowness is probably because every cell is rendered as an individual little HTML5 canvas (two, one for off state and one for on state).

Some browsers have no problem at all with so many little canvases, but in firefox it causes a slowdown.

A solution that will hopefully be faster is to make one big canvas instead of many little ones, and blit the cells on it every frame instead of using CSS visibility to swap between on and off states for each cell. Such rewrite of the rendering engine is a todo.