Firescope is a Firefox add-on that allows you to search HTML and CSS reference material directory from Firebug.

After installing you will see a Reference tab in Firebug. From there you can search for HTML Elements, HTML Attributes, and CSS Properties. It shows you what the element is and also it’s compatibility in Internet Explorer 7, Firefox 3, Safari 3, and Opera 9.

It’s a pretty simple add-on, but works as intended. For beginning developers I can see it helping quite a bit and for advanced developers as the HTML spec changes it will give a quick view of what works and what doesn’t.