To read: Mozilla Firefox Release Notes

Installation :
yum --enablerepo=remi update firefox xulrunner5
Notice : this RPM is close of firefox 5 one from fedora 15.

This package needs xulrunner5 which install beside default one (you can remove xulrunner2, if present).

RPM are available for fedora 11 to 14.

For fedora 15, it's on the road to official updates. If you have installed the beta version from my repository, you should remove it to avoid any conflict.