http://www.computing.co.uk/ctg/news/2158826/mozilla-developing-ie10-rival-windows
Mozilla developing IE10 rival for Windows 8
http://www.brianbondy.com/blog/id/129/
A bit of background on Windows 8 Metro and Firefox
It turns out that on Windows 8 there are 3 types of applications:
Classic desktop applications
Metro applications
Metro style enabled desktop browsers
Firefox will fall into the third category, meaning you can run Firefox as a desktop application, and you can run it as a Metro application. Supporting the Metro side of things will require a lot of new code, so this is a very large project.