var am = Cc["@mozilla.org/network/http-auth-manager;1"].getService(
  Ci.nsIHttpAuthManager
);
am.setAuthIdentity(
  "http",
  "mochi.test",
  8888,
  "basic",
  "testrealm",
  "",
  "mochi.test",
  "user1",
  "password1"
);