As the author writes:
Download the X-Ray.XPI
Once 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.
While I create webpages all the time, there’s something about writing a piece of code you have to install that’s very satisfying. Even better was finding that it was actually something I’d find helpful myself.
For version 0.5 the current list of supported tags is:
h1, h2, h3, h4, h5, h6, p, ol, ul, li, dt, dd, font, div, span, blockquote, pre, a, b, i, strong, em. If some other people find this extension useful, I’ll probably add support for more tags, and possibly experiment with adding some other enhancements as well.