One of the greatest strengths of Firefox is its powerful extension system, which gives third-party developers the ability to expand the browser's capabilities. Although it is often blamed for slowing browser performance as well, this extensibility delivers a lot of value to Firefox users. Mozilla is looking to build on this particular strength with the introduction of Jetpack, a new API designed to make Firefox add-ons easier to code and deploy.
Currently in its 0.1 version, Jetpack is still "unpolished, unfinished, and still highly prototyped" according to its developers, who expect to complete a 1.0 release within the next six months. You can try out a few code samples, and read more about the new API, on the Jetpack developer site.