It was written for Firefox 3 as it uses no images. The red ball is in fact just a div with some Firefox-only css (-moz-border-radius is my best friend). Here is an excerpt from the help to get you started:
The goal of the game is to keep the ball inside the box using the paddle. The paddle will follow your mouse movements from inside the playing area. To start the game, just click anywhere inside the box.
As usual, for posterity’s sake, here is a screenshot. To try the game out for yourself, go to http://carduner.net/pong