--- a/toolkit/themes/shared/close-icon.inc.css
+++ b/toolkit/themes/shared/close-icon.inc.css
@@ -19,10 +19,9 @@
   fill-opacity: 0.2;
 }
 
 .close-icon > .button-icon,
 .close-icon > .button-box > .button-icon,
 .close-icon > .toolbarbutton-icon {
   width: 20px;
   height: 20px;
-  border-radius: 2px;
 }

--- a/toolkit/themes/shared/icons/close.svg
+++ b/toolkit/themes/shared/icons/close.svg
@@ -1,7 +1,7 @@
 <!-- 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/. -->
 <svg xmlns="http://www.w3.org/2000/svg" width="20" height="20">
-  <path fill="context-fill" fill-opacity="context-fill-opacity" d="M0 0h20v20H0z"/>
+  <rect fill="context-fill" fill-opacity="context-fill-opacity" width="20" height="20" rx="2" ry="2"/>
   <path fill="context-fill" fill-opacity="context-stroke-opacity" d="M11.06 10l3.47-3.47a.75.75 0 0 0-1.06-1.06L10 8.94 6.53 5.47a.75.75 0 1 0-1.06 1.06L8.94 10l-3.47 3.47a.75.75 0 1 0 1.06 1.06L10 11.06l3.47 3.47a.75.75 0 0 0 1.06-1.06z"/>
 </svg>