How odd. I tried searching for the issue real quick, but this is all I found:

Clip paths are supported by all major browsers except for Internet Explorer, but one factor takes Firefox out of the equation as well. Firefox supports the technology, but only as an SVG-referenced path, meaning that alteration of the coordinate would need to be done outside of CSS. I’m sure someone with more of a developer’s brain could find a way to get this working across all browsers, but what I value in this project is, first, that it’s unabashedly experimental and, secondly, that it works across most mobile devices with good performance.

From this Smashing Magazine article.

He doesn't outright say that it's impossible on Firefox, and a commenter said that it opens in Firefox instantly for them but not in Chrome. It works fine for me in Firefox but the pieces transition between animals isn't there.

EDIT: Could be a plugin issue of somesort?