Mozilla has been working on a Firefox specifically designed for the new Windows 8 interface for quite a while. The first early builds are now months old, but progress has been slow it seems.
Luckily, no one is buying Windows 8 computers or upgrading to Windows 8, so Mozilla can take it slow.
Now, Mozilla is satisfied enough with the performance of the feature that it's enabling don't-call-it-Metro UI support by default in Firefox 21
, currently in the Nightly channel.
Whether this means Firefox 21 will ship with Metro UI support once it reaches the stable channel is another matter, most likely that's not going to happen, judging by the state of the current Firefox for Metro UI.
If you're curious to see what Firefox for Metro UI, but not curious enough to install Windows 8, Mozilla's Paul Rouget has some "official" screenshots
Or, you could just pull out your Android phone or tablet and install Firefox, you'll be seeing pretty much the same thing. In the meantime, enjoy.