Design Principles Behind Firefox OS Explained
At MozCamp Warsaw, a presentation was given on the design principles behind the core Firefox OS experience. Layering of applications (if you're wondering why the Firefox mobile interface has that weird curve by the tab control, you'll find answers here), an emphasis on content over visual frills for their own sake, consistent iconography, and clean typography dominate.
Here is the entire design document in one sentence:
Make the app look like an Android app, make the home screen look like iOS (but circles instead of rounded squares).