Damn, this should work, we do support Firefox 21 ! It's probably a regression in the demo itself and not the engine. We'll fix it asap, thanks for the tip !

Ok, try reloading the page. We use requirejs but not the domready! plugin. So there seems to be a race condition with the body loading. Sorry for that, we'll fix it asap.

I tried again now, still getting the same error.

... but now it worked.

I get the same error on FF Nightly (24).