Sure, these packages (from the French Les RPM de Remi) for Firefox 5 will work on any Red Hat Enterprise 6 derived distro, including CentOS. But I first read about them on the (increasingly active) Scientific Linux Forum. The rpms were actually compiled for Fedora 12, the release from which RHEL 6 was developed.
xulrunner5, the prerequisite package. Don’t bother to try removing this first, just do an rpm -Uvh (removing it could wind up whacking 21 or so other packages). For i386 just substitute in the url.
firefox-5. Remove firefox-3 before installing. Ignore the warnings about Fedora 12 being end-of-life. For i386 just substitute in the url.
When matched with 64-bit versions of Adobe’s Flash Plugin and the Iced Tea Web Plugin this release of Firefox is noticeably faster than what came before, and I’m quite happy with it. One of the really nice things about moving up to SL 6 has been the ability to finally run later and better releases of software than I could under RHEL/CentOS/SL 5.