The most recent Nightly versions of the Firefox web browser ship with the first system add-on released for the browser. Firefox Hello, the browser's realtime chat component, has been turned into a system add-on.
Mozilla announced in mid-2015 that it would turn specific features of Firefox into system add-ons in the near future. The idea behind the move was to make updates to these features independent from browser updates.
If Mozilla wanted to update Firefox Hello in previous versions of Firefox for instance, it would have to release an update for the whole browser. The new method speeds up the delivery of updates for components turned into system add-ons
Another benefit of the add-on approach is that Mozilla may deliver features to a sample of Firefox's userbase for testing purposes.
Firefox 45 or newer systems ship with the first system add-on already. One core difference between regular add-ons and system add-ons is that system add-ons cannot be removed from the add-ons manager.
In fact, the current implementation does not even list Firefox Hello in the add-ons manager of the browser.
The about:support page however lists it as an extension. System add-ons are not installed in the user's profile directory but in the installation directory of Firefox.
The directory differs depending on the operating system used and the version of Firefox that is installed on the system. Windows users find the Nightly directory under C:\Program Files (x86)\Nightly\browser\features, and the regular Firefox directory under C:\Program Files (x86)\Firefox\browser\features instead.
Please note that the latter won't list system add-ons until it reaches version 45 at the very least. In fact, the features directory does not even exist yet in any version of the web browser but the most recent Nightly versions.
Since system add-ons are placed in the installation directory, they are enabled for all profiles used on the system automatically.
While Mozilla may list system add-ons in the Add-ons Manager eventually, giving users controls to disable them, no such feature exists currently.
You may delete these add-ons however in the features directory, and the add-ons are removed from Firefox as a consequence. I cannot say yet if this is permanent, or if updates will add them again to the browser.
I will keep an eye on this and update the article if the add-on is added again to the browser at a later point in time.
Update: The system add-on is added again when the browser updates.
It would be great in my opinion if Mozilla would give users full control over system add-ons. Those who don't want to use certain features should be able to disable or even delete them permanently from the browser.
The next feature that is going to be turned into a system add-on is Pocket. (via Sören Hentzschel).
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