Use o complemento do Firefox Aardvark (ou de aqui )
To use Aardvark, just right-click on the page and select Start Aardvark from the popup menu. As you move the mouse over the page, you will see a red rectangle around the current block element, along with a label showing its type, and if they exist, its id, class, and style.
You can then press various keys on the keyboard to do things, for instance W to navigate wider (as shown in the image), R to remove the element, I to isolate the element (that is, remove everything else on the page), V to view formatted source of that element, U to undo, Q to quit aardvark, and H for help, that is, a list of the 14 or so available keystrokes.