gecko/toolkit/themes/windows/global/textbox-aero.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/. */

%include textbox.css

@media (-moz-windows-default-theme) {
  textbox html|*.textbox-input::-moz-placeholder {
    font-style: italic;
  }
}