One Firefox add-on that I’ve been playing around with recently is Firescope from SitePoint.
Firescope is an extension to Firebug, which is one of the most useful tools for debugging websites, and essentially adds reference material for HTML and CSS to Firebug.
It shows you code examples of all HTML and CSS elements, how well the various browsers support it (Full, Partial, Buggy or None), and gives you links to more detailed information about each element on the SitePoint reference sites:
This is a welcome addition to Firebug, and will certainly save me time checking the exact syntax of those HTML and CSS elements that I don’t use very often. Even though I have the SitePoint Ultimate Reference guides to HTML and CSS both on my desk at work, and in PDF format on my desktop.