The Web Developer extension adds a menu and a toolbar to the browser with various web developer tools. You can view source, validate the page, hide images or show selected information about images, such as which ones are missing their alt text. One of the features I most often use is real-time display of the effects of editing the stylesheet and the ability to hover over elements and see their position in the stylesheet, eg (much abbreviated): html > body > div#content > p > span.booktitle.
You start Aardvark with a Control Click (right click for Windows users) on the page and choose “Start Aardvark” from the menu. As you move your mouse around the page a red rectangle frames each element under the cursor. You will also see a little yellow caption showing the HTML element type and its class or id if they exist. You can then press keys to do various things, such as delete the selected element from the page, isolate the element, or move the selection rectangle outward to the containing element.
[Aardvark Via: Six Log]