Boot to Gecko aims to create a complete, standalone operating system for the open web.
You can read more about B2G here:
This fork of B2G aims to restore and maintain B2G for the modern web. We believe that with more HTML5 web apps than ever before, Firefox OS would be more than capable of being a contender in not only the Mobile space, but also a viable competitor against Chrome OS given enough time and effort.
Not sure if this'll go anywhere, we'll see. Currently it's either untouched, or barely modified. Would not recommend building now unless you wish to contribute. (Issue reports are welcome, as well as Pull requests.)