In the past I have used the instructions here to run more recent versions of firefox.http://wiki.centos.org/TipsAndTricks/Firefox4onCentOS5
This has worked fine and basically involved a one time coming up with an unsupported copy of libstdc++.so.6 for local use by Firefox 4 and above.
What has changed with Firefox 8 is that they now include a usable copy of libstdc++.so.6 with the tarball, making it a standalone working build that only needs to be extracted for use.
Nice to see that a RPM is being made available now. I see one at REMI for CentOS 6, but not for CentOS 5. Maybe I missed it, maybe the one for CentOS 6 will work anyway?
Edit: The REMI package for CentOS 6 does not work on CentOS 5 (dependencies).