I can cause the segfault to happen simply by calling the following function
before attempting to follow a pdf link.  Note, sometimes it takes a couple
of attempts on the link for the failure to manifest.

  function query_eps () {
      var eps = Cc["@mozilla.org/uriloader/external-protocol-service;1"]
          .createInstance()
          .QueryInterface(Ci.nsIExternalProtocolService);
  }

Nothing more than simply QI'ing nsIExternalProtocolService.  What is going on here?