I'll show you how to install the Java runtime plug-in for Mozzilla Firefox in Ubuntu's terminal command line program by downloading the self-extracting .bin installer from Java.com and running it in Terminal and verifying it on Java.com.

Once you downloaded the JavaInstaller.bin file to your default download folder in Ubuntu. You need to be login as an administrator root user.

Open up Terminal in Applications menu>Accessories>Terminal

In terminal, type these commands 

cd /Home/Username/downloads/

chmod a+x NameOfJavainstaller.bin

./NameOfJavainstaller.bin (press enter)

Yes

ls

Go back to Java.com to verify your install, and let Mozilla firefox install the needed plug-ins for it.

Congrats now you installed Java.