Jetpack is cool, really cool. All browsers should allow plain javascript/html web extensions. I hope it gets standardized by the W3C in the future.
What I don't like is web pages asking me to install them when I visit them.
There should be just a link to the extension, like old xpi. A link, not a popup. If you want, you can use JSX as the entension and instruct the browser to behave accordingly.
Mozilla, please get rid of the extension installer pop up and the {link rel="jetpack" href="myextension.js"} non-standard aberration.