You really should look into using yum or up2date to bring your system current because the current updated version for FC4 is;
With that said to answer your question, Thomas Chung has three files available; http://www.fedoranews.org/tchung/firefox/220.127.116.11/
Download these three files and the latest released version of firefox;
Place the downloaded firefox-18.104.22.168.tar.gz file along with firefox.desktop and firefox.png in someplace like /usr/src/redhat/SOURCES
Place the firefox.spec file in someplace like /usr/src/redhat/SPECS
Edit the firefox.spec file to reflect the newer version, save the file when done. Then create the new installable version from the /usr/src/redhat/SPECS location typing something like;
rpmbuild -bb firefox.spec
Having the rpm-build-4.4.1-21 rpm package installed is required.
Read the output from the build command to findout where the installable rpm package is.
FYI: thunderbird-22.214.171.124 is also available, the building of the installable rpm file is almost exactly the same, all you need the spec file;
the thunderbird.desktop and thunderbird.png files;
They go in the same relitive places and then use the rpmbuild command from the /usr/src/redhat/SPECS location.