To read: Mozilla Firefox Release Notes

Installation :
yum --enablerepo=remi update firefox xulrunner6
Notice : this RPM is close of firefox 6 one from fedora 15/16.

This package needs xulrunner6 which install beside default one (you can remove xulrunner2 and xulrunner5, if present).

RPM are available for fedora 11 to 14.

Should also be soon in fedora 15 updates