gnome-shell favorites doesn't allow non-default-path program.
Download firefox nightly for linux 32/64.
http://nightly.mozilla.org/
extract it in your home
if [32bit]
$ wget
http://ftp.mozilla.org/pub/mozilla.o...x-i686.tar.bz2
if [64bit]
$ wget
http://ftp.mozilla.org/pub/mozilla.o...x86_64.tar.bz2
$ cd
$ pwd
/home/<user_account>
if [32bit]
$ tar jxf firefox-14.0a1.en-US.linux-i686.tar.bz2
if [64bit]
$ tar jxf firefox-14.0a1.en-US.linux-x86_64.tar.bz2
$ cd /usr/bin
$ su
$ mv firefox firefox_fedora
$ ln -s /home/<user_account>/firefox/firefox firefox