An online copy of O'Reilly's 2002 book Creating Applications with Mozilla. It focused on pre-1.0 versions of Mozilla, so details about the application structure has changed, but the XUL elements are relatively stable.
Another Douglas Crockford video. This time Douglas talks about the Document Object Model, about how Java failed, and JavaScript evolved thanks to DHTML and DOM. He talks about how to use DOM to traversing and manipulating elements in an HTML document, walking the DOM, making elements, innerHTML, as well as Events, memory leaks. He talks about the cracks in DOM, and how we must be prepared to back off when we hit the browser limits of DOM.
Christian Heilmann's excellent guide to writing unobtrusive JavaScript. Takes the reader from the inline JavaScript world right through to a clean dynamically attached events with all the JavaScript code in external files. Covers DOM Scripting. This is the key reference to unobtrusive scripting.
Mark Pilgrim's essential online book about developing Greasemonkey scripts. Excellent starting point for Greasemonkey developers. Covers everything from setting up a Greasemonkey scripts to common DOM coding idioms, as well as a brief rundown of XPath, and case studies of Greasemonkey scripts.
Jeremy Keith describes unobtrusive JavaScript by talking about separating content, style and behaviour.