How do I get the Modern UI of Firefox in Windows 8?

Ok, firstly, there seems to be a quirk in Windows 8: only the default browser can be launched with Modern UI. I have tested with IE, FF and Chrome on the Enterprise Evaluation and they will all launch the Desktop version unless set as default. Additionally, as soon as a browser is set as the default handler for HTTP/HTTPS, any other running Modern UI is instantly closed. More information is available at this question.

So, I have it running fine. There were a few things I did, any of which may be responsible:

• Installed an older Firefox build (FF 18), specifically the installer executable

• Set as default for all via Control Panel => Programs => Default Programs => Set Default Programs => Nightly

  
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• When the Modern UI version launched, but hung with a blank screen, I closed it via Task Manager, set IE back to default, and set Nightly to default again. At this point, the Modern UI version of FF worked.

• Installed the latest elm build (FF 19, 24-Oct-2012 06:06), again from the installer executable. Restarted.

So, now I have Nightly running with Modern UI:

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It doesn't quite work right yet. Using a shortcut to the executable (Might be in Program Files (x86))

"C:\Program Files\Nightly\firefox.exe" -metrodesktop

with that new parameter opens up the Metro version of Firefox Elm on the regular desktop.

Chances are this will be fixed in future builds

I'm not exactly sure what the problem is here, I just installed the nightly build myself without issues.

It didn't seem to activate the metro version (when launched from the start screen) until after a reboot, but from then on it worked just fine.

Keep in mind though it is still in NIGHTLY status, and many bugs should be expected.

PS: same thing happened with metro chrome. On first install it was desktop only, but once reboot, metro worked.