I'm experiencing a weird issue with canvas tags using webgl. When I open some pages with webgl, my top toolbar (back/forward buttons, awesome bar and all the rest) is mirrored in the bottom of the canvas element instead of showing the expected content. Focusing any other window causes the mirrored image to be replaced with the actual content. I have tested this with several webgl pages, the problem only appears on some of them, and I have no idea why. An example page that does show the problem: http://alteredqualia.com/three/examples/webgl_cubes.html. I have also tested this under a default profile with none of my addons installed, and still experienced the problem. I'm running Firefox 25, under OSX 10.6. Screenshot here: http://imgur.com/TZcABYN I've searched around and not found any reference to a problem like this. Is this an unreported bug?