Lola will let you find HTML/XHTML blocks visually just by moving around the mouse pointer over the page, and modify them "on the fly" from your browser quick and easily.
To use it, just click on the "Lola" icon (find it at the lower tool bar of your browser) to enter the "select" mode. Move the mouse pointer over the page you want to inspect or modify, and every HTML/xHTML block will be highlighted to you.
Note: The Lola add-on is cross-platform and it works on Mac OS X, Windows and Linux.