Back out changeset ddd02164f9f7 (Bug 842618) now that Bug 845341 is fixed r=Neil over IRC.
See:
Bug 842618 - SeaMonkey contains PrivateBrowsingUtils.jsm, but it throws an exception if used
Bug 845341 - PrivateBrowsingUtils.permanentPrivateBrowsing can throw if the autostart pref does not exist