I was getting information on the latest java update before installing so I also did a search for octet stream. Octet stream is a java applet by Sun Microsystems, the developers of Java. The application is safe to down load. I believe that Mozilla will handle java, but it has to be enabled, but you can disable it when you do not need java support.
If you don't have good protection on your system, you should keep java disabled when not needed. IE makes enabling/disabling java easy to do, but I am not familiar with Mozilla's features.