You'll need to manually create a symbolic link to the java plugin, which is housed in usr/lib/java/jre/plugin/i386/ns7/libjavaplugin_oji.so in /usr/lib/mozilla/plugins as root. The reason it should go into /usr/lib/mozilla/plugins is that the directory is set via our init scripts to be the default browser plugins directory. ;-)
With that said, make sure that Firefox is not running. Type the following commands at a terminal prompt:
rm -rf libjavaplugin_oji.so
ln -sf /usr/lib/java/jre/plugin/i386/ns7/libjavaplugin_oji.so .
Hope this helps you.