I'm not 100% sure about using the atrpms site myself............. they have a habit of changing a lot of the 'default' packages. BTW I'm using the firefox package to reply here.
$ cat /etc/*release
Red Hat Enterprise Linux Client release 5 (Tikanga)
You should also consider adding the epel yum repo also;
http://fedoraproject.org/wiki/EPEL
I have the FC6 yum repo's added myself, but like the rest of the third party repo's they are disabled by default. This keeps things easy to manage when I need something from a third party repo I add it to the yum command, for example;
yum install something --enablerepo=fc6*
The FC6 repos are fairly safe to add, RHEL5 (and by default CentOS5) is after all based on the work done for FC6. Here's a copy of my FC6 yum config file;
[fc6-base]
name=Fedora Core 6 Updates
mirrorlist=http://mirrors.fedoraproject.org/mirrorlist?repo=core-6&arch=$basearch
enabled=0
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-fedora-extras
gpgcheck=0
[fc6-updates]
name=Fedora Core 6 Updates
mirrorlist=http://mirrors.fedoraproject.org/mirrorlist?repo=updates-released-fc6&arch=$basearch
enabled=0
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-fedora-extras
gpgcheck=0
[fc6-extras]
name=Fedora Core 6 Extras
mirrorlist=http://mirrors.fedoraproject.org/mirrorlist?repo=extras-6&arch=$basearch
enabled=0
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-fedora-extras
gpgcheck=0
You might want to also use the thunderbird 2.0.0.4 package (if you use thunderbird) from the F7 or Fedora Development.