XUL Reference is a Firefox extension that allows you to highlight names of xpcom/xul/xbl components and interfaces in webpages, 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. For instance, if you select the word nsIMsgFolder, then the right click menu will show 'XUL Reference (XPCOM interface)' and clicking on it will open the reference page for nsIMsgFolder in a background tab.
The selection doesn't have be be totally accurate, these will all work:
The data files with all the names ship with the extension, but they can be updated by going to Tools - XULReference and there hitting the 'Reload data' button. Then it will fetch all component names again from xulplanet.com
· Firefox 2.0 - 3.0.*
What's New in This Release:
· Updated for Firefox 3.0 compatibility and updated with latest reference files.
NOTE: xulplanet.com is no longer being updated so this reference will start to become more and more out of date. This will probably be the last version of this addon, if xulplanet.com closes it will be removed from AMO.