I don't do this very often and keep forgetting how to do it.
So here is a note for the next time.
Installing Java for Firefox is not as easy on Linux as on some other operating systems. This is the process for a Debian based distribution like Lubuntu.
The Java plugin can be installed for a given user, or all users. I'll describe the process for all users.
Download a Linux plugin version suitable for your computer from www.java.com.
Open file manager, the select Tools > Open Current Folder as Root
Move the java file (e.g. jre-7u51-linux-i586.tar.gz) to /usr/java
...create a java folder if you don't already have one.
In file manager (still as root) navigate to /usr/java and select Tools > Open Current Folder in Terminal
In terminal type:-
tar zxvf jre-7u51-linux-x64.tar.gz
...but use the current version file name
...this will unzip the file and create a new folder with the expanded contents.
Still in the file manager as root, open this new folder and navigate to:
...but use the name of your new jre-xxxx folder in place of the example.
Now look for either an i386 folder or an amd64 folder (or something else) depending upon your processor type. In this folder you should find a file called: libnpjp2.so
Copy the path to this file (e.g. /usr/java/jre1.70_51/lib/amd64/libnpjp2.so)
Now navigate to the Firefox plugins folder, using your file manager, still as root:-
If this folder already contains a previous libnpjp2.so link, this should be deleted.
Select Tools > Open Current Folder in Terminal
Create a symbolic link by pasting in the saved file path prefixed by ln -s, like this:-
ln -s /usr/java/jre1.70_51/lib/amd64/libnpjp2.so
Now start Firefox
In the address bar type:-
...and ensure your new Java(TM) Plug-in is listed with the correct version and path.