Learn how to dig into a WordPress site with Firebug or other web inspectors and see how it’s put together. Investigate specific elements and learn what CSS rules are being applied to them. Live edit CSS for quick and easy theme building. Make faster sites by checking the size and speed of your pages and files. Find and fix JavaScript bugs or theme and plugin conflicts.

Tutorial: http://jleuze.com/inspecting-wordpress-with-firebug/