Firefox users can no longer deactivate system add-ons. Mozilla has decided not to allow users from disabling system add-ons. If you don’ know, in the latest Firefox release, you can visit about:performance page and disable the system add-ons such as Pocket and Hello, latter one soon to be removed from Firefox 49.
Though about:performance, which shows performance of add-ons and web pages in real-time, doesn’t allow uninstall of a system add-on — when you click uninstall button, nothing happens — but it does allows user to deactivate the hidden add-on, Mozilla has separated sometime back after integrating. Now when you visit this page in Firefox 50 or later versions, like the uninstall button, disable button also doesn’t work.