View generated source – important if your script modifies the original DOM of your page
Display Element Information – shows all the properties of an element in a convenient popup. To use, press Ctrl+Shift+F or select menu Information/Display Element Information
Edit CSS – lets you edit the CSS selectors in real-time, and shows you the results – almost as you type
MeasureIt. Lets you find the height and width of any element or area of the page, in pixels. Helps diagnose CSS issues, for example, when you use combinations of margin, width, padding and border attributes.
Codeburner from Sitepoint, one of my favorite online journals (www.sitepoint.com). Integrates with Firebug to provide comprehensive cross-reference information on HTML and CSS. Best feature: