The idea is neat, but the playback option doesn't show me the drawings in Firefox 12.0 on Windows XP (cube computer).
Yeah there are some bugs in Firefox. I only had time to develop in Chrome and I think FF has some issues with Javascript setTimeout(), but it very well could be on my end. I'll try to pinpoint when I get a chance.
If you click play, then jump to a different spot in the video, you'll see the canvas redraw and it should work.