Dont ask me why, there is no JRE shipped with Ubuntu (CD size story)
Just type:
sudo apt-get install openjdk-7-jre icedtea6-plugin
And you will get the Open JRE and the plugin for Firefox
Sources:
http://blog.sudobits.com/2012/04/21/how-to-install-java-jre-on-ubuntu-12-04/
http://superuser.com/questions/162096/how-to-configure-applet-plugin-for-openjdk-in-firefox-on-linux