[BillGoates]

In FireFox the problems are either with the timer or how fast a screen is drawn.

On an old laptop, changing the opacity, or set a different color of the same amount of divs takes about 50ms. The next timeout or interrupt, is on average 200ms later. The first screen draw is the slowest with 350ms.

I used absolute position for the divs, so that the browser doesn't have to recalculate the page after each update.