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The game doesn't seem to work in Firefox; the background is black and the tombstones vibrate wildly around the screen.

oh ... thats a first.  You can use the native version for windows or linux (whichever you have), to play the game.

Which OS and firefox version did you use?

This was with the latest version of Firefox on Windows 8. It might have been caused by one of the four ad blockers I have, though. I switched to Chrome and it ran fine.