If you're a web developer, or even if you have an interest in the way web pages work, then Firefox has a few built-in features that should be of interest.

Within the Tools menu, you will find a JavaScript Console and a DOM Inspector. The JavaScript Console monitors any messages or warnings generated by the scripts on the current page. You can also use the console to log all of the warnings and messages in a session. The DOM inspector is a two-paned tool that can be used to browse the hierarchy of different elements on a page. You can view and edit Document Type Declarations, scripts, and stylesheets within the Inspector.