CodeBurner for Firebug

Features (click on the images for a larger version)

Begin with the search box.	You can search for HTML elements, HTML attributes, CSS properties, CSS selectors, or CSS at-rules (or any combination). The search happens live as you type, so usually you'll only have to type one or two letters to get the results you want.
Information is detailed but succinct.	Search results have information about browser compatibility, standards-compliance, and a brief summary of usage. There are also links to further information on the SitePoint Reference web site.
Context-menu options.	Right-click in the search results and you'll be given two options, one of which is for more information online, and the other is to show a code example for the selected item.
Practical code examples.	Code examples open in the side-panel of the Reference, HTML or CSS panels, and are practical examples with summaries, to show you how the item can be used. Code examples are also editable and easy to copy (simply double-click in Firefox 3 to Select-All).
Adding to the HTML context menu.	In the HTML panel you can right-click on any element name, attribute name or attribute value (include while it's being edited) and you'll have two options — to Look up the item you clicked on, or to Show Code Example for that item.
Adding to the CSS context menu.	Similarly, in the CSS panel you can right-click any property name or value, for the same options.
Searching for a selected element.	While in the HTML panel, if you right-click on the tag name of the element that's currently selected (usually highlighted in dark blue), the context menu will say Look up selection; clicking that will then do a special search — for the selected element, all the attributes that are defined for it, and all the CSS properties, selectors and at-rules that currently apply to it. The special search also happens automatically — whenever you click the Reference tab, the panel will default to showing you data for the currently-selected element.
Code Examples follow Inspector.	While using the Inspector to select different elements, if the Code Example side-panel in the HTML panel is open, it will automatically bring up an example for whatever element is being inspected.
Adding to the crumbtrail options.	Likewise, the DOM crumbtrail responds to clicks to show code examples, if the Code Example side-panel is open. It also has context-menu items the same as the main HTML panel, with options to look-up the selected item or view a code example for it.