Rated 4 out of 5 stars

I'm not sure why Mozilla themselves didn't write a better description, but here is what I got from the project's official page:

"In short, Jetpack is an API for allowing you to write Firefox add-ons using the web technologies you already know."

Source: https://jetpack.mozillalabs.com/

This review is for a previous version of the add-on (0.2.1).