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.
The general idea for Jetpack is to allow users to create add-ons using technologies they already know, such as HTML, CSS and JavaScript, thus lowering barriers to would-be developers. It also enables developers to create code packages that can be toggled on and off without the end user having to restart their browser and should address performance concerns by resulting in more lightweight extensions. This comes as somewhat of a preemptive move against Google, which has downplayed add-ons over performance with its Chrome browser.
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.