gecko-dev/dom/base/test/script_postmessages_fileList.js

var { classes: Cc, interfaces: Ci, utils: Cu } = Components;
Cu.importGlobalProperties(["File"]);

addMessageListener("file.open", function () {
  var testFile = Cc["@mozilla.org/file/directory_service;1"]
                   .getService(Ci.nsIDirectoryService)
                   .QueryInterface(Ci.nsIProperties)
                   .get("ProfD", Ci.nsIFile);
  testFile.append("prefs.js");

  File.createFromNsIFile(testFile).then(function(file) {
    sendAsyncMessage("file.opened", { file });
  });
});

addMessageListener("dir.open", function () {
  var testFile = Cc["@mozilla.org/file/directory_service;1"]
                   .getService(Ci.nsIDirectoryService)
                   .QueryInterface(Ci.nsIProperties)
                   .get("ProfD", Ci.nsIFile);

  sendAsyncMessage("dir.opened", {
    dir: testFile.path
  });
});