Looks like the first hurdle is that it's relying on the deprecated document.registerElement API from an early draft of the Web Components specs, which prevents the app from booting in Firefox.
Either the polyfills aren't being shipped or their feature detection is broken, because I get "document.registerElement is not a function" in Firefox Nightly with a spoofed UA string, and I get marginally further if I turn on our experimental native support for Web Components: http://imgur.com/a/TgnrQ
Polymer's polyfills don't work very well, for what it's worth. Sometimes they get confused if things don't load from the network in the order they expect and start eating up gigabytes of RAM and all your CPU...
Edit: Paul Kinlan reports that Earth is using PNaCl (https://twitter.com/Paul_Kinlan/status/854351477506277378), which means it fundamentally can't run outside of Blink.