i think you are supposed to create a symbolic link from the plugin to the firefox plugins directory. the library stays where it is...
remove libjavaplugin_oji.so from /usr/lib/firefox-1.0.7/plugins
ln -s /usr/java/jre1.5.0_06/plugin/i386/ns7/libjavaplugin_oji.so /usr/lib/firefox-1.0.7/plugins/libjavaplugin_oji.so
then it should work.
also are you sure its the ns7 version??