FireScope is a Firefox add-on that integrates with Firebug, to extend it with reference material for HTML and CSS.

The extension’s core functionality is centered around a new Reference panel, which contains a search tool for looking up HTML elements, attributes, and CSS properties.

The extension also hooks into context-menus in the HTML and CSS panels, the DOM crumbtrail, and the Inspector, adding options to look up a selected item (ie. search for it in the Reference panel) or to view a code example.

Download FireScope at http://tools.sitepoint.com/firescope