Firefox for Windows 8 is in effect two web browsers: the desktop version that looks and feels much the same as your current Firefox, and a new Modern UI Windows app designed for Windows 8's tablet-friendly interface. They are of course the same program, but separated by the somewhat schizophrenic nature of Windows 8 (on PCs and laptops at least).
Firefox for Windows 8: performance
In terms of speed, within the desktop mode Firefox for Windows 8 recorded exactly the same scores in our Sunspider test as did Firefox 16 - completing the test in around 200ms every time. But the Modern UI app didn't fair so well, recording scores of around 260ms. We saw something similar when using our less well specified Windows 8 tablet. In desktop mode Firefox completed the test in around 420ms (the same as Firefox 16), but within the Modern UI environment the scores slipped to around 460ms. In general browsing we didn't notice the difference, but it is there. It'll be interesting to see if similar issues affect other programs that span the divide between old and new Windows.
Using Firefox for Windows 8 across all four instances was stable, and we saw none of the memory-hogging behaviour of which Firefox is sometimes accused.
Firefox for Windows 8: UI changes
There are some interesting UI changes consistent to both versions of Firefox in this iteration. New tabs offer a three-column view of your bookmarks, recent history, and downloads, presented as rectangular icons that follow a similar style to the rest of the Windows 8 interface. On top of the Windows you'll find a unified bar for URLs and searches.
When you're on a web page, Firefox switches to a full-screen view without URL bar or any options - it's not dissimilar to IE 10's approach in Windows 8. You have to right-click to bring up the address bar, and right-click again to show all open tabs. Testing Firefox on our Windows 8 tablet we had to swipe down from the top of the screen for the address bar, and once again to see the tabs.
You can configure the browser to show the address bar and all tabs at all times, but it's worth perservering - sites look great without what Microsoft refers to as the 'chrome' that clutters up modern browsers.
Right-clicking/swiping down also brings up a few other options in a menu bar at the bottom of the screen. Here you can jump to the downloads list, find a word on the page, open a page on the desktop, zoom in or out, create a bookmark, or pin the page to the Start screen.
Firefox for Windows 8 supports sharing via the Charms bar. Find a page you like and you can share it with the world through your Twitter or Facebook app of choice. Of the other Charms Windows 8's universal search function is not yet supported, but should be by the time Windows 8 launches.
You can download Firefox Metro UI Preview here.