FireScope is a Firefox add-on that integrates with Firebug and provide HTML and CSS reference information with your Firefox browser. Firebug is a revolutionary plug-in and famous add-on among web developer/ designer and FIreScope extend it with reference material for HTML and CSS.
The add-on is being developed by SitePoint an online resource for Web professional market, specifically Web Developers and Designers.
The extension’s core functionality is centered around a new Reference panel, which contains a search tool for looking up HTML elements, HTML attributes, or CSS properties (or any combination). The search happens live as one type i.e. auto suggest to get the reference results.
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.
The add-on comes worthy for designers/ developer when one wants to learn CSS/ HTML while developing some app/ website using same on the go. As right-click in the search results give two options, one of which is for more information online, and the other is to show a code example for the selected item it provide pretty good reference material on the fly.