FIREBUG
Use the F12 key to activate/deactivate the feature of this addon.  Use it to investigate HTML, CSS and JavaScript on any page you visit.  The "Inspect" feature is particularly impressive, allowing you to visually associate HTML elements to the respective code.

WEB DEVELOPER
This toolbar allows you to pick apart any website.  You can disable CSS (embedded, linked, inline, or All), disable scripts, disable cookies; you can outline HTML elements, including tables, images, divs, anchors, links; you can validate code.  And more!  This addon makes it really easy to visualize what code has what effect on any website!