Mozilla Add-ons Builder is a Web-based add-on SDK that let you quickly and easily develop an extension for Firefox using common web technologies. Get started by simply pressing the Create Add-on button and typing in your code!
Don't want to start from scratch? Use a variety of APIs in the core library to add functionality to Firefox, including widget and context-menu. The Add-ons Builder basically tacks on a rather pretty (albeit nascent and not-very-useful) 'file explorer' interface to Bespin. It works, but only just. While SDK helps you find libraries, there's no auto-complete for library methods. The tutorial suggests you look at the source code of other add-ons for inspiration, but there's no way of searching through them -- just a big, page-after-page directory.