First, let's look at the Java option. What is Java, and how does it affect you on the Web? Java is just a programming language that enables certain functions on websites. By default in Firefox, this option is checked off, but Firefox does NOT come with Java already installed. If you come to a site that requires Java, Firefox will prompt you to install it.
You don't have to install Java. It's not vital to your Web experience, but it's a nice feature. I have it on my machines and I've never had any problems. If you would like to install Java direct from the source,here is the direct Java source.