Mozilla, the organization behind various popular open source projects, has released a preview build of Firefox for Windows 8 ‘Metro’ UI (officially, Modern UI). The current build can be downloaded from here (available for both, the 32-bit and 64-bit OS) to test the features of upcoming Metro version of the browser. In past, Mozilla devs had released a metro prototype based on the Fennec XUL code.
It installs itself as nightly build of Firefox which is different from the regular nightly build of Firefox. So, if you use nightly build of Firefox, for your development and testing purposes, you should keep this in mind. After successful installation, its ‘new’ launcher appears on the Start screen of the Windows 8. This new icon is different from that of the regular nightly build, as shown in the screenshot below.
(PS: We tried the above preview build on Windows 8 RTM, and unfortunately it failed to start there.)