You can use packages from Fedora 7 to do this. As a regular user, open a terminal and do:
Stay in the same directory and su to root, then:
rpm --import http://download.fedora.redhat.com/pub/fedora/linux/releases/7/Everything/i386/os/RPM-GPG-KEY-fedora
yum remove yelp
yum localinstall fedora-bookmarks-7-1.noarch.rpm
yum localinstall firefox*
yum localinstall libbeagle-0.2.16.2-5.fc7.i686.rpm
yum localinstall yelp-2.18.1-5.fc7.i386.rpm
If you're running 64 bit ("uname -m" returns x86_64), replace "i386" in the URLs above with "x86_64"
It should be noted that these might have some effect on your system stability as they're not really "made for" CentOS. I've had no problems so far though. Also, Firefox 2 is too new to be considered *stable*.