Uninstall OpenJDK (optional but I don't recommended mixing Sun and OpenJDK Java on the same machine)
sudo apt-get purge openjdk-6-jre openjdk-6-jre-headless
Create Mozilla plugins directory (if not present)
cd ~/.mozilla
mkdir plugins
Create a soft link from the plugins directory to the 64 bit plugin object libnpjp2.so
cd ~/.mozilla/plugins
ln -s /home/chris/jdk1.6.0_30/jre/lib/amd64/libnpjp2.so .
Now restart Firefox.