If you can't see anything, or the
only interesting thing that happens here occurs when you click on the
example (it turns bluish until you click off it), your Firefox build
isn't set up to display canvases. Specify --enable-cairo
as a build-time option and recompile to see the expected effect.
Your build also has to be pretty new — older than a week may
not cut it.
The created effect is the random placement within a 450x450 area of two randomly rotated/scaled images (see directory listing) with varying levels of transparency. Clicking on the page and image trigger different transparency and color effects. Get a build; my description didn't do it justice.
There's also a crudely-made video of what happens, but it really is crude. The changes in the animation correspond to me clicking on the canvas and clicking on the page. Clicking on the canvas changes the image outputted to it, while clicking the page changes the color filter over the canvas itself. (Roughly.)