If you’ve got an older Windows machine and can’t see WebGL graphics even with a nightly build of Firefox, and you’re getting weird stuff like error codes mentioning
0x80004005 (NS_ERROR_FAILURE) in your error console after the Canvas 3D initialisation messages, it may be because your graphics drivers are out-of-date — even worse, it might be that your graphics card is so old that it doesn’t support a recent-enough version of OpenGL to support WebGL in hardware.
There is a solution — you can enable software rendering, which means that the WebGL content will be slow (because it’s not using the graphics card’s hardware acceleration) but you should be able to see at least roughly what’s going on. To do this, go to this post by Vladimir Vukićević, and follow the instructions in the paragraph starting “For those of you on Windows who don’t have an up to date OpenGL driver”.
I’m saying this from bitter experience… so far I’ve been writing my WebGL on a desktop machine and on a newer laptop belonging to someone else. I decided to try it out on my own laptop the other day, and got the errors as described. A driver update didn’t help with WebGL (though it did improve the performance of some OpenGL stuff), and the only solution I found was software rendering.