From the 'future of add-ons' files:

Mozilla's Jetpack project was updated over the weekend to version 0.3, improving on a number of key features. Jetpack is Mozilla's vision for a next generation add-on technology - add-ons are very important to Mozilla with over a billion total installs.

From my vantage point the biggest change in Jetpack 0.3 are the improvements to the SlideBar. Basically SlideBar is Mozilla's Jetpack term for a browser side bar.

"Slide bars have been very well received, with dozens of Jetpacks implementing on top," the Jetpack 0.3 release announcement states. "Based on the feedback we got, we've dramatically revised the API. We've also added new features, like the ability for a slide bar to notify the user of an update in a fun but non-obtrusive way."

Makes a lot of sense to me.

I'm still not clear one how the existing massive user community of Mozilla add-ons will transition to Jetpack over time. Currently Mozilla says that they have had over 60,000 that currently have Jetpack. That number isn't too shabby for an early effort, but still has some ways to go before it can eclipse the existing method of browser add-ons.

The real key in my opinion is when/if Mozilla will decide to formally integrate Jetpack into Firefox 3.6. They've already decided to integrate Ubiquity (as Taskfox). Adding Jetpack is likely just a matter of time - until the APIs settle and core functionality is stable.