Firefox is one of the latest versions of the Mozilla .
It is currently available in two repositories - Remi and Spot.
See the next steps if you want install Firefox 4.
First , go on super user shell.
# wget http://repos.fedorapeople.org/repos/spot/firefox4/fedora-firefox4.repo -O /etc/yum.repos.d/fedora-firefox4.repo
Now, you can install Firefox 4
# yum --enablerepo fedora-firefox4 install firefox4
That is all.