They used to also use the unreleased version 0 of shadow DOM for building the Polymer UI, which - being a Chrome-only prototype - understandably didn't work on Firefox, and therefore instead used a really slow Javascript polyfill to render its UI.
I haven't checked on it lately, but I imagine they must've changed at least that by now.


There's a bunch of extensions that allow you to switch user-agent easily, I personally use this one, it includes a list of known strings to choose between as well.