Today’s preview marks the beginning of Mozilla community testing for the Firefox Metro browser designed from the ground up for Windows 8.
As the above say's Mozilla is ready for you to try the Metro version of Firefox:

The preview includes a new Metro style Firefox Start page, support for Firefox Sync, Metro touch and swipe gestures, integration with Windows 8 “charms”, and a simple but powerful Australis interface that is streamlined, modern, and beautiful. Over the coming weeks and months, we’ll be adding more features, tightening up Windows integration, improving performance and responsiveness, and finishing up all the necessary work to deliver a first class Firefox experience for Windows 8.

This is an early preview and we know there are bugs and missing features, so please don’t hesitate to share your feedback and file any bugs you find. Stay tuned to this channel for further updates.
Download:

Firefox Metro Preview for Windows 8 | Future Releases