Excerpt:
Jetpack is being described as “an exploration in using Web technologies to enhance the browser (e.g. HTML, CSS and Javascript), with the goal of allowing anyone who can build a Web site to participate in making the Web a better place to work, communicate and play.” In other words, if you know how to develop for the web, you should be able to build a Jetpack app. Developers can write code using Mozilla’s web-based Bespin environment and the popular Firebug extension.