Firefox Normandy Preference Rollout System

Normandy Pref Rollout is a feature that allows Mozilla to change the default value of a preference for a targeted set of users, without deploying an update to Firefox. There are then several mechanism we can use to turn this flag on to enable the feature off-train, Normandy pref rollout is a simple, safe, and featureful way. A version change, removing an add-on, changing a preference, or any other change in targeting criteria will not automatically remove a user from a rollout.

Source: wiki.mozilla.org