context:   observe: function (aSubject, aTopic, aData)  {    if (aTopic == "shell:desktop-background-changed") {      var setDesktopBackground = document.getElementById("setDesktopBackground");      setDesktopBackground.hidden = true;            var showDesktopPreferences = document.getElementById("showDesktopPreferences");      showDesktopPreferences.hidden = false;      var os = Components.classes["@mozilla.org/observer-service;1"]                         .getService(Components.interfaces.nsIObserverService);      os.removeObserver(this, "shell:desktop-background-changed");    }  },
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