Mozilla announced in October that it would release a Metro version of Firefox for Windows 8 users. Company released a buggy release preview, code named "Elm" back then. Now nightly build users can get a taste of less buggy version of Firefox Modern UI. It will run along side your current version of Firefox, so no need to worry that it will overwrite your stable browser.
If you are on the Firefox Nightly channel and you have a Windows 8 device, your Firefox update should deliver a Metro Firefox tile to the far right end of your Windows Start screen.
There's plenty of work still to do, but it's stable enough that we're ready for more and more regular testing, says Asa Dotzler.
(Express version is online installer, download the Standard version for Windows)
The build is labeled as "21.0a1" and there are versions for Windows, Mac, Linux and Android.
For accessing the Metro (Modern UI), you will need to set Firefox as your default browser and set "http" to open within the browser.