Firefox is similar enough to Netscape that you can follow the Netscape instructions. And I've never ever seen those "install plugin" dialogs work, not even when I used Netscape under Linux.
Assuming you've done the RPM download from Sun's site and installed it with
rpm -ivh name-of-bin-you-downloaded
you can just create a symlink to libjavaplugin_oji.so in /usr/lib/firefox/plugins .
ln -s /usr/java/jre1.6.0/plugin/i386/ns7/libjavaplugin_oji.so libjavaplugin_oji.so
If you can't find libjavaplugin_oji.so after you install the RPM package you can issue
and then type
to find it.
After you create your symlink you may have to close and re-open all instances of Firefox in order for java to show up in about