X-Ray for Firefox

See the tags on a page without viewing the sourcecode.
When you applied to a page it can help you see how the document was constructed without having to go back and forth between the sourcecode and the page in your browser. Is that list made of li, dd or p elements? Is that an h3 tag or just some bolded text? X-Ray shows you what's beneath the surface of the page.

Add to Firefox
https://addons.mozilla.org/en-US/firefox/addon/1802