sudo apt-get install sun-java5-jdk
Accept the licence agreement that appears.

Set Sun’s JVM as default Java version:
sudo update-java-alternatives -s java-1.5.0-sun

Edit /etc/jvm and move /usr/lib/jvm/java-1.5.0-sun to the top of JVMs offered:
gksudo gedit /etc/jvm

install the firefox plugin:
sudo apt-get install sun-java5-plugin
then click here to test it