Issue 5973 - broken icons on Firefox v50-53

skin/desktop-options.css

 /*
  * This file is part of Adblock Plus <https://adblockplus.org/>,
  * Copyright (C) 2006-present eyeo GmbH
  *
  * Adblock Plus is free software: you can redistribute it and/or modify
  * it under the terms of the GNU General Public License version 3 as
  * published by the Free Software Foundation.
  *
  * Adblock Plus is distributed in the hope that it will be useful,
  * but WITHOUT ANY WARRANTY; without even the implied warranty of
@@ skipping 365 matching lines @@
 }
 
 [data-validation] .floating-input input:focus:invalid ~ .attention::before
 {
   background-image: url(icons/attention.svg);
 }
 
 [data-validation] .floating-input input:valid ~ .attention::before
 {
   top: 0.8rem;
-  background-image: url(icons/checkmark.svg#approve);
+  background-image: url(icons/checkmark.svg#approved);
 }
 
 [data-validation] .floating-input input ~ .error-msg
 {
   margin-top: 0.5rem;
   color: #C11D27;
   display: block;
   visibility: hidden;
 }
 
@@ skipping 1044 matching lines @@
 
 #hide-notification::after
 {
   background-image: url(icons/delete.svg#secondary);
 }
 
 #hide-notification:hover::after
 {
   background-image: url(icons/delete.svg#secondary-hover);
 }