Dave, First, confirm that Java is installed.
cd /usr/java/
you should have a directory "jre1.5.x.x.x" or whatever the current version. That directory should have some stuff in it:
<SNIP> [ahull@hal jre1.5.0_02]$ ls bin COPYRIGHT lib man README Welcome.html CHANGES javaws LICENSE plugin THIRDPARTYLICENSEREADME.txt <!SNIP>
If it does not, then Java is not installed yet. I recommend not downloading the RPM.
One you get it installed, then you have to copy the plugin to the proper directory in Firefox.
a symbolic link would do this nicely.
ln -s n -s /<java dir>/plugin/i386/ns7/libjavaplugin_oji.so /<firefox dir>/plugins/
Then enable the java checkbox in firefox preferences and you're set.
The java website has some decent instructions too: http://java.com/en/download/help/5000010500.xml#selfextracting
Andrew