If you only want to run Firefox 3 as one user all you need to do is download the .bz2 file, unpack ie and then you'll get a directory called Firefox. In that directory you'll find an ex executable called firefox. You run that. If you want to run it as more than one user then you'll have to put it somewhere everyone can access it and give it suitable permissions.
I'm fairly certain that Firefox 3 will not run under RHEL 4 due to a dependency on GTK 2.10 or higher and I suspect RHEL 4 doesn't have a new enough version of GTK. I don't know about RHEL 5.
You may find this thread helpful http://www.linuxquestions.org/questi...efox-3-658306/