--- a/browser/themes/windows/browser-aero.css
+++ b/browser/themes/windows/browser-aero.css
@@ -160,19 +160,21 @@
     color: white;
   }
 
   #toolbar-menubar:not(:-moz-lwtheme) {
     text-shadow: 0 0 .5em white, 0 0 .5em white, 0 1px 0 rgba(255,255,255,.4);
   }
 
   /* Vertical toolbar border */
-  #main-window[sizemode=normal] #navigator-toolbox:not(:-moz-lwtheme)::after,
-  #main-window[sizemode=normal] #navigator-toolbox > toolbar:not(#toolbar-menubar):not(#TabsToolbar):not(:-moz-lwtheme),
-  #main-window[sizemode=normal] #navigator-toolbox:-moz-lwtheme {
+  #main-window:not([customizing])[sizemode=normal] #navigator-toolbox:not(:-moz-lwtheme)::after,
+  #main-window:not([customizing])[sizemode=normal] #navigator-toolbox > toolbar:not(#toolbar-menubar):not(#TabsToolbar):not(:-moz-lwtheme),
+  #main-window:not([customizing])[sizemode=normal] #navigator-toolbox:-moz-lwtheme,
+  #main-window[customizing] #navigator-toolbox::after,
+  #main-window[customizing] #navigator-toolbox > toolbar:not(#toolbar-menubar):not(#TabsToolbar) {
     border-left: 1px solid @toolbarShadowColor@;
     border-right: 1px solid @toolbarShadowColor@;
     background-clip: padding-box;
   }
   #main-window[sizemode=normal] #browser-border-start,
   #main-window[sizemode=normal] #browser-border-end {
     display: -moz-box;
     background-color: @toolbarShadowColor@;