I think that you may be linking to the wrong file: The link seems to be pointing to the firefox plugins directory, rather than the actual jave plugin.
Otherwise, the output of ls -l should look something like this:
ls -l /usr/lib/firefox/plugins
lrwxrwxrwx 1 root root 39 2005-06-23 10:41 libjavaplugin.so -> /usr/lib/java/plugin/ns7/libjavaplugin_oji.so
The above is from my computer, the paths on yours may be different.
I hope this helps