I had an issue where the default plugin was not being recognized by Firefox 2. The first thing I did was upgrade the JRE to version 6 by running an apt-get in terminal:

Next I checked to make sure the upgrade went as planned by checking the installed Java version using the following terminal command:

However, it would seem the root of the problem was the Mozilla plugin directory did not exist. In the end I was able to fix the plugin for Firefox by recreating the plugins directory in my Home folder and creating a link to the Java 6 JRE libjavaplugin_oji.so using the following steps (again in terminal):mkdir .mozilla/plugins cd .mozilla/plugins ln -s /usr/lib/jvm/java-6-sun/jre/plugin/i386/ns7/libjavaplugin_oji.so

This was all it took to get the Java plugin functioning again in Firefox 2.