This is because the regular central Nightly channel builds is only the desktop version.

You need a build that has Metro support.

http://ftp.mozilla.org/pub/mozilla.org/firefox/nightly/latest-elm/firefox-20.0a1.en-US.win64-x86_64.installer.exe

There is this running thread at mozillaZine about the Metro builds. http://forums.mozillazine.org/viewtopic.php?f=23&t=2304983

Amazing!!! Thank you very much But still one thing is I have to make a choice between 64-bit Firefox and Metro Firefox, :(

Whoops sorry not sure why I had the 32-bit version posted. There is the 64-bit version linked now.

Not sure about the Metro builds, however there is a chance that the 64-bit central Nightly builds maybe stopped for a while (not discontinued as many articles incorrectly posted) as they are not even officially supported and were mainly for catching regressions/breakage as it is possible to break Win64 builds without breaking the official Win32 builds. Bug#814009 (is a example for reading as bugzilla is not a discussion forum). The Win64 builds are still a work in progress despite what many may think as many are those who think 64-bit is better because it is a bigger number when Win32 builds actually works better on 64-bit Windows currently.

I have changed from 64-bit to 32-bit, anyway I get the metro back, although it is still in progress.

However, a strange thing keeps happening to me when I wanna update the Nightly. It says "The partial Update could not be applied. Nightly will try again by downloading a complete Update".