Firefox is a browser for the internet, an alternative to Safari or Internet Explorer. The main selling-point (for me) of Firefox is its flexibility: Firefox comes with a powerful interface for creating and modifying the user interface, giving the user control of his or her browsing experience. Not only does Firefox come with this ability, but there is a large community of developers who use it, and share their work in the form of extensions. So if you haven't already, get Firefox. It's free and open-source, and I couldn't function without it.
Using a variety of resources, most of which were found at the Mozilla Developer Center, I have built and packaged a simple Firefox extension. It prints "Hello, World!" to the lower-right corner, in the status bar. It's perhaps a little unambitious, but it's a necessary step in learning a new skill, and I'm very pleased with it. You can try it for yourself here. You will need Firefox 3.0 or later to use this extension.
The structure of Firefox is a little complicated, but quite powerful. The user interface is written in a language called XUL (read "zool"), and Firefox is a XUL runtime environment (although I may oversimplify unintentionally). One amusing result of this design is that you can load Firefox as a XUL application within another instance of Firefox. To do this, simply open a new tab and paste the text chrome://browser/content/browser.xul in the address bar.