From Fedora Project Wiki
Firefox 5 is now available as an official update for Fedora 15. Tom Callaway has a repository available for Fedora 14 users that want to use the latest release.
Download http://repos.fedorapeople.org/repos/spot/firefox5/fedora-firefox5.repo and place it in /etc/yum.repos.d.
su -c 'wget -P /etc/yum.repos.d/ http://repos.fedorapeople.org/repos/spot/firefox5/fedora-firefox5.repo'
Use PackageKit or run the following command,
su -c 'yum install firefox5'
Note: You might want to remove any other Firefox versions on your system. Although Firefox 5 is parallel installable, running it together with other versions is not recommended.