I just ran across another cool extension for Firefox called Aardvark.

(if you aren't a web developer you can skip this one, it probably won't interest you...)

Aardvark will let you move your mouse over different elements of a web page and it will show you information about the element.

You can expand and contract the selection that you are currently viewing (if, for example, you have selected a table cell, pressing W will select the table row...), remove elements from the page, isolate an element, colorize elements (this one could be pretty useful), view the source of your selection, and more...

Very cool!