(http://i50.tinypic.com/2qvflfo.jpg)
If you can't enjoy Firefox Metro version in Windows 8, simply follow these simple steps to set it as default browser app and your problem will be fixed:
1. If you are on Start Screen, type default and Windows will show "Default Programs" in search results page. Click on it to open Default Programs window.
Alternatively, you can open it from Control Panel.
2. Now click on "Set your default programs" link given at the top.
3. Now select Nightly from the given list and click on "Set this program as default" option.
(http://i46.tinypic.com/24yql9e.jpg)
4. As soon as you click on the option, Windows will show a notification asking you to select the default browser for opening http links. Actually its a notification for default browser app for Start Screen.
(http://i50.tinypic.com/2uprbeo.jpg)
Select Nightly from the given list of browser.
5. That's it. Now you can enjoy Firefox Metro version without any problem.
PS: If you don't get the Metro app notification mentioned in step 4, open Internet Explorer and set it as default browser. Now Windows will show the app notification and you can select Nightly as default browser app.