The java plugin is named 'libjavaplugin_oji.so' and there should be a link to that file in your
'$HOME/.mozilla/plugins' ------- for current user or
'/opt/firefox/plugins' --------- for all users, assuming that firefox was installed in that directory, as you said.
So a proper command to set up the link would be:
ln -s /usr/java/jre1.5.0_06/plugin/i386/ns7/libjavaplugin_oji.so ~/.mozilla/plugins/
ln -s usr/java/jre1.5.0_06/plugin/i386/ns7/libjavaplugin_oji.so /opt/firefox/plugins
I guess this path to 'libjavaplugin_oji.so' is right, otherwise you have to adjust it, by searching for the name and use the correct path.