It is building towards an actual future layout. IIRC, once the changes are stable enough they're pushed to the main nightly builds. Edit: Sorry, I was unclear,

It is building towards an actual future layout. IIRC, once the changes are stable enough they're pushed to the main nightly builds. Edit: Sorry, I was unclear, it is an official branch and not just a personal fork. Some links: http://msujaws.wordpress.com/2012/03/15/mozillas-ux-nightly-build-of-firefox/ http://msujaws.wordpress.com/2012/03/15/mozillas-ux-nightly-... https://wiki.mozilla.org/UX_Branch https://wiki.mozilla.org/UX_Branch