I was trying the site out in Firefox 3 on Ubuntu and it appeared to run fine. I also tried it on Firefox 2 on a Windows XP machine and it was pretty much identical. I could only ever play in the favicon. The instructions say that if you press Enter it should open in a canvas display but I could not get that to work on either machine.
Here is a short video of me trying to play Defender of the Favicon. Not easy to do but it is a creative use of the space. Perhaps a different game would be eaiser to play as a favicon. Pac-Man comes to mind or even Asteroids maybe.
This is a unique approach to creating a favicon and deserves to be recognized this week as the favicon of the week!