Make Firefox act like a small database server that is smart about RDF data.
The W3C's Resource Description Framework (RDF) standard is one of the more complex XML standards. RDF is an application of XML, but it makes little use of the XML tag hierarchy. Instead, it turns any specified tags into facts. Facts are stored in memory separate from the DOM tree of the RDF content. This hack shows how to use facts. Firefox has both scripting and direct XML support for facts.
What is Firefox RDF good for? It's a very general mechanism for managing any kind of data, whether that data fits in an XML hierarchy or not. The output of an SQL query is an obvious example.
If you are new to RDF, be selective about the material you lean on. Much RDF literature is not suited for beginners. The book Rapid Application Development with Mozilla (Prentice Hall PTR) is one source of a digestible tutorial. An alternate approach is to learn a little Prolog. RDF and Prolog are both examples of propositional calculus (a field of mathematical logic). On top of these ideas, you also need a bit of experience scripting Mozilla XPCOM components.
Very briefly, RDF is a more general way of representing data than relational tables. RDF is concerned with making statements about things. Each statement is comprised of three pieces of information: the thing we're making the statement about, a property of the thing, and the value of that property. In RDF parlance, this is called a triple ...