[EDIT] If anyone is wondering why it is rendering poorly in Firefox with Resist Fingerprinting enabled, that's because this page seems to serve the game from a random CDN on a different host. https://v6p9d9t4.ssl.hwcdn.net/html/8469963/index.html in my case. Because the canvas is attempting to read pixels, which can be used for fingerprinting, Firefox is blocking this. But because it is being done in an iframe, there…
It sounds like you have privacy.resistFingerPrinting enabled, an experimental feature which is disabled by default: https://support.mozilla.org/en-US/kb/firefox-protection-agai...
I'll note that apart from a handful of sites where it causes additional checks and occasional breakage (noted them in another comment below), I'm really happy with that setting. I leave it on by default and only have to break out a fingerprinted Firefox profile on rare occasion.