--- a/browser/themes/shared/browser.inc.css
+++ b/browser/themes/shared/browser.inc.css
@@ -31,17 +31,17 @@
 
 #navigator-toolbox::after {
   content: "";
   display: -moz-box;
   border-bottom: 1px solid var(--toolbox-border-bottom-color);
 }
 
 :root[customizing] #navigator-toolbox::after {
-  display: none;
+  border-bottom-style: none;
 }
 
 /* Bookmark toolbar */
 
 #navigator-toolbox > toolbar:not(#toolbar-menubar):not(#TabsToolbar):not(#nav-bar) {
   overflow: -moz-hidden-unscrollable;
   max-height: 4em;
   transition: min-height 170ms ease-out, max-height 170ms ease-out;