On Tuesday, our preliminary Metro Firefox development work arrived at mozilla-central, the source code repository that feeds the Firefox Nightly channel. This means that if you are on the Firefox Nightly channel and you have a Windows 8 device, your Wednesday 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. If you're a Firefox or Windows developer and you'd like to get involved, please let me know.