Extensions are built using CSS, Javascript and HTML, which means that more people can build them and install them without restarting the web browser. What Mozilla is ultimately planning for Jetpack isn't clear, but we'd place a wager on some of the technologies making their way into Mozilla's future mobile browser.
Today, Mozilla has launched the Jetpack Gallery, which will make it easy for users to find and try out Jetpack add-ons created by the community.
In the six months since Jetpack's introduction, the Mozilla Labs team has put a lot of work into beefing up Jetpack's API and capabilities. Just yesterday, Jetpack 0.6 was released, which adds the ability to add and modify menus and have secure preferences.
Looking at the Jetpack Gallery, you won't find many Jetpacks (just 19 at the time of publication), but looking at what types of Jetpacks are available, it's easy to see the potential of this sort of add-on system.