gecko-dev/toolkit/content/menulist.css

/* This Source Code Form is subject to the terms of the Mozilla Public
 * License, v. 2.0. If a copy of the MPL was not distributed with this
 * file, You can obtain one at http://mozilla.org/MPL/2.0/. */

@namespace html url("http://www.w3.org/1999/xhtml"); /* namespace for HTML elements */

html|*.menulist-editable-input {
  -moz-appearance: none !important;
  background: transparent ! important;
  -moz-box-flex: 1;
}