Q:

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

Mozilla mentions

Elm is the experimental repository where most of our Metro development work has been happening.

Now, I grabbed the latest nightly from the Elm repo, installed it & then despite starting it from the start screen, I was still provided the desktop version.
Is there a way I can get the Metro Modern UI of Firefox in Windows 8? Do I need to add some extra command line arguments or something?

A:

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

A:

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
Click for full size
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:
Click for full size
Click for full size

