Have you installed the Java plug-in yet? If not, the easiest way to do that is
to open a terminal window and execute the following (after the dollar sign
prompt):

$ sudo apt-get install sun-java6-plugin<enter>

This will download the Sun Java plug-in for version 6.0, and will also create
all of the necessary symbolic links to make it work. You will need to restart
Firefox after this.

To verify the installation, open a new Firefox window (or tab) and type
"about:plugins" in the address bar. This will display your Firefox plugins.
Scroll through this window and look for the section titled "Java(TM) Plug-in
1.6.0_14".