XUL Reference allows you to highlight names of xpcom/xul/xbl components and interfaces in Web pages, then right-click and go straight to its page on xulplanet.com, which is the best xul reference out there. This is more than a search engine plugin because the extension knows the names of all XPCOM components and interfaces, XUL elements, XBL Elements, events, scriptable objects, and common properties/attributes/style-classes of the XULElement. If you highlight one of those, then the right click menu will show you what type of object you've selected and open a background tab with the reference page for that object. This version is the first release on CNET Download.com.
||Windows XP/Vista, Firefox: 2.0 - 2.0.0