System add-ons are a relatively new feature of the Firefox web browser. The main idea behind the feature is to separate these add-ons and the functionality they provide from Firefox's core.
The benefit of doing so is that updates can be delivered to these add-ons without pushing out an update for the Firefox web browser itself. They are updated like any other add-on on the system (which is also independent from core Firefox updates).
They work in most regards like any other add-on installed by the user, but cannot be controlled by the user currently.
The two system add-ons Firefox ships with currently are Pocket and Hello, two rather controversial features.
http://www.ghacks.net/2016/03/29/mozilla-system-add-ons-controls/