Issue 5973 - broken icons on Firefox v50-53

skin/mobile-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 214 matching lines @@
 {
   border-color: #BBBBBB;
 }
 
 button.remove
 {
   width: 36px;
   height: 36px;
   padding: 0;
   background-color: #099DD1;
-  mask: url(mobile/trash.svg) center/19px no-repeat;
+  mask: url(mobile/trash.svg#default) center/19px no-repeat;
 }
 
 ul + button
 {
   flex-shrink: 0;
   width: 100%;
   min-height: 46px; /* equal to min-height of list item */
   padding: 15px 20px; /* based on margin and padding of list item */
   border: 1px solid #BBBBBB;
   text-align: start;
@@ skipping 111 matching lines @@
   flex-shrink: 0;
   width: 13px;
   height: 11px;
   margin: 10px;
   content: "";
 }
 
 #dialog-recommended ul li.installed::before
 {
   background-color: #BBBBBB;
-  mask: url(mobile/checkmark.svg) center/cover no-repeat;
+  mask: url(mobile/checkmark.svg#default) center/cover no-repeat;
 }
 
 #dialog-recommended > button
 {
   border-width: 1px 0 0 0;
   text-align: center;
 }