You need to symbolically link the Java plugin module to your mozilla plugin directory (/usr/lib/mozilla/plugins). Eg if you installed Java in /opt/java, you should see a file named "libjavaplugin_oji.so" in the /opt/java/plugin/i386/ns7 directory. The following command will link the plugin for you:
ln -s /opt/java/plugin/i386/ns7/libjavaplugin_oji.so /usr/lib/mozilla/plugins/
and then restart Mozilla.
(you'll need to do this from a terminal window as root and obviously change /opt/java to wherever you installed Java)