Mozilla Labs developers are cranking out the Jetpack releases. In less than a month, they've got another release for you.
It seems like it was May 27th when Jetpack SDK 0.4 was released - it was. Now the Mozilla Labs developers have released Jetpack SDK 0.5.
The new version of the Mozilla toolkit includes access to tabs, a request API, a localization API and something called "load/unload reasons."
It's just a lot easier than the current plugin framework based on XUL.