userContent.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/. */

/*
 * Edit this file and copy it as userContent.css into your
 * profile-directory/chrome/
 */

/*
 * This file can be used to apply a style to all web pages you view
 * Rules without !important are overruled by author rules if the
 * author sets any.  Rules with !important overrule author rules.
 */

/*
 * example: turn off "blink" element blinking
 *
 * blink { text-decoration: none ! important; }
 *
 */

/*
 * example: give all tables a 2px border
 *
 * table { border: 2px solid; }
 */

/*
 * example: turn off "marquee" element
 *
 * marquee { -moz-binding: none; }
 *
 */

/*
 * example: make search fields on www.mozilla.org black-on-white
 *
 * @-moz-document url-prefix(http://www.mozilla.org/) {
 *   #q { background: white ! important; color: black ! important; }
 * }
 */

/*
 * For more examples see http://www.mozilla.org/unix/customizing.html
 */

/*
 * Display the corresponding URL when a hyperlink is hovered, like Chrome.
 */

a[href]:not([content]):after {
  visibility             : hidden;
  font-family            : Leelawadee, "Segoe UI", Candara, "Bitstream Vera Sans", "DejaVu Sans", "Trebuchet MS", Verdana, "Verdana Ref", sans-serif !important;
  font-size              : 9pt !important;
  font-weight            : 400 !important;
  font-style             : normal !important;
  text-align             : left !important;
  color                  : rgb(111, 111, 111) !important;
  background             : rgb(223, 223, 223) !important;

  position               : fixed !important;
  top                    : calc(100% - 1rem - 4px) !important;
  bottom                 : 0px !important;
  left                   : 0px !important;
  padding                : 2px 9pt !important;


  content                : attr(href) !important;
  white-space            : nowrap !important;
  opacity                : 0;
  z-index                : 999999999 !important;
  height                 : 1rem !important;
  line-height            : normal !important;

  -moz-transition        : all 0.4s ease;
  -moz-transition-delay  :0.10s;


}
a[href]:hover:after {
  visibility: visible;
  opacity   : 1;
}