To enable Java Runtime Environment plugin on Firefox, try this:
sudo aptitude install sun-java6-bin sun-java6-jre sun-java6-plugin
Then, make a symbolic link to libjavaplugin_oji.so
(which is located in /plugin/i386/ns7/ directory of Java on my Java-1.6.0 installed system) in your Mozilla Plugins directory.
To locate Mozilla plugins directory, simply type:
updatedb
locate firefox | grep /plugins$
Then, restart Firefox and that’s all.