The design work hasn't even started; this prototype is built with the same front-end code and theme as Firefox for Android. (Actually it's using a mish-mash of two different Android themes, because of a quirk in the way the prototype is built.)
The icons in the prototype were all designed for Android, and all will be replaced by real Windows/Metro icons in the final product.
[Usual discosure: I am a mobile Firefox developer.]
The rendering engine code is shared across all platforms (Android, Linux, Mac, Windows, etc.).
The "mobile" front-end code is currently used by Firefox for Android/Maemo/MeeGo. It's separate from the "desktop" front-end code used by Firefox on Windows/Mac/Linux.
The icons in the prototype were all designed for Android, and all will be replaced by real Windows/Metro icons in the final product.
[Usual discosure: I am a mobile Firefox developer.]