I had the same problem when I created a symbolic link to the wrong plugin.
For Java 1.5.0 it should be in the ns7 directory.
Type the following from the Firefox plugin directory:
Code:
ln -s /usr/java/jre1.5.0/plugin/i386/ns7/libjavaplugin_oji.so
For Java 1.4.2 it should be in the ns6-gcc directory.