After earlier prototyping Mozilla in 2011 introduced the Jetpack addon SDK in an effort to simplify Firefox addon development, and allow installing and using addons without requiring a Browser restart.
Addons were offered by adding a HTML
link tag to the documents
<head> data; e.g.
<link rel="jetpack" href="kissaki_trim.js" />.
Now that Jetpack addons have been deprecated, this does not have any effect on current versions of Firefox.