I wanted to run a Java applet from Firefox. I was using Firefox on Ubuntu. I am using Open JDK 6. You need libnpjp2.so browser plug-in for that. However, Open JDK does not have this plug-in. So, had to install Sun Java plug-in:
sudo apt-get install sun-java6-plugin
Then you need to go to the firefox plug-in directory and make a hard link to the libnpjp2.so library. (You need to close firefox before making the hard link)
ln -s /usr/lib/jvm/java-6-sun-22.214.171.124/jre/lib/i386/libnpjp2.so .
You will see the plug-in listed in Tools > Add-ons > Plugins tab. You can enable or disable any time. (QuickJava Firefox extension provide a nice little tool to enable/disable on the fly.)
Hope that helps.