The X-Ray extension for Firefox will let you see HTML tags without looking at the source code. This is a very handy tool!
One installed the X-Ray command is available by right-clicking as well as in the Tools menu. When 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.
New Firefox Extension: X-Ray [Design Meme]