From Mozilla Labs:
Add-ons Builder is a Web app for quickly and easily building Firefox add-ons using common Web technologies. The add-ons you build can make changes to the browser via libraries of modules that provide APIs for accessing Firefox functionality. The core library is available by default to all add-ons and provides the most commonly-requested APIs, including APIs for accessing tabs, storing data, displaying a widget, and making network requests. Add-ons can also use libraries contributed by other users of the site.
If you are comfortable with a command line and prefer working locally, the Jetpack SDK gives you a development environment on your own computer.