Since it's a prototype we're focusing on fast iterations using webkit specific CSS properties and such without worrying about cross browser support. I think it works okay in Firefox, but we haven't tested it and don't guarantee it for this phase.

It requires Chrome because putting it up in the Chrome Web Store allows users to download and install it, which removes concerns about latency (since the assets total 7MB, we don't want the game art loading in fragments). It also allows us to point to a single place where people can get the game. And Google is in charge of hosting and bandwidth, something else we don't want to be worrying about right now.

If your concerns lie with Chrome being a Google product, I believe you can install it in Chromium as well, though I haven't tried it myself.