FireBug is a must-have Firefox extension for anyone interested in web site design. It allows you to explore web pages (your own to debug or others to learn from) an an intuitive and very useful way. FireBug's author describes the extension like this:
FireBug lets you explore the far corners of the DOM by keyboard or mouse. All of the tools you need to poke, prod, and monitor your JavaScript, CSS, HTML and Ajax are brought together into one seamless experience, including a debugger, an error console, command line, and a variety of fun inspectors.