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.
You can’t disable pocket in about:performance page
READ: Firefox 45: Mozilla makes Firefox Hello as System Add-on
