- Download and install the java plugin with the command 'sudo apt-get install sun-java6-plugin'
- Navigate into that sub-directory and find the path to 'libjavaplugin_oji.so'. It will be somewhere like '/usr/lib/jvm/java-6-sun/jre/plugin/i386/ns7/'. You could also search for 'libjavaplugin_oji.so' with the command 'sudo find /usr -name libjavaplugin_oji.so'. If that find command doesn't find anything then try 'sudo find / -name libjavaplugin_oji.so'.
- Open a terminal window within your Firefox plugins directory (in Mint 9 it is '/usr/lib/mozilla/plugins/') and create a simlink to 'libjavaplugin_oji.so'. Your command will be something like 'sudo ln -s /usr/lib/jvm/java-6-sun/jre/plugin/i386/ns7/libjavaplugin_oji.so libjavaplugin_oji.so'
- Exit and restart Firefox. From Firefox 3.6 on there is no "Java" checkbox under Tools -> Options -> Content. Java is turned on and off through Tools -> Add-ons