--- a/browser/themes/windows/browser-aero.css
+++ b/browser/themes/windows/browser-aero.css
@@ -1,14 +1,13 @@
 /* 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/. */
 
 %filter substitution
-%define toolbarShadowColor hsla(209,67%,12%,0.35)
 %define glassActiveBorderColor rgb(37, 44, 51)
 %define glassInactiveBorderColor rgb(102, 102, 102)
 
 @media (-moz-os-version: windows-win7) {
   @media not all and (-moz-windows-classic) {
     #main-window[sizemode="normal"] > #tab-view-deck > #browser-panel > #navigator-toolbox > #toolbar-menubar {
       margin-top: 1px;
     }
@@ -255,17 +254,17 @@
       -moz-appearance: -moz-win-exclude-glass;
     }
   }
 
   @media (-moz-os-version: windows-win8) {
     /* Artificially draw window borders that are covered by lwtheme, see bug 591930.
      * Borders for win7 are below, win10 doesn't need them. */
     #main-window[sizemode="normal"] > #tab-view-deck > #browser-panel:-moz-lwtheme {
-      border-top: 1px solid @toolbarShadowColor@;
+      border-top: 1px solid @glassShadowColor@;
     }
   }
 
   @media (-moz-windows-default-theme) {
     #main-menubar > menu:not(:-moz-lwtheme) {
       color: inherit;
     }
 
@@ -285,36 +284,36 @@
   }
 
   /* Show borders on Win 7 & 8, but not on 10 and later: */
   @media (-moz-os-version: windows-win7),
          (-moz-os-version: windows-win8) {
     /* Vertical toolbar border */
     #main-window[sizemode=normal] #navigator-toolbox > toolbar:not(#toolbar-menubar):not(#TabsToolbar):not(:-moz-lwtheme),
     #main-window[sizemode=normal] #navigator-toolbox:-moz-lwtheme {
-      border-left: 1px solid @toolbarShadowColor@;
-      border-right: 1px solid @toolbarShadowColor@;
+      border-left: 1px solid @glassShadowColor@;
+      border-right: 1px solid @glassShadowColor@;
       background-clip: padding-box;
     }
 
     #main-window[sizemode=normal] #navigator-toolbox:not(:-moz-lwtheme)::after {
-      box-shadow: 1px 0 0 @toolbarShadowColor@, -1px 0 0 @toolbarShadowColor@;
+      box-shadow: 1px 0 0 @glassShadowColor@, -1px 0 0 @glassShadowColor@;
       margin-left: 1px;
       margin-right: 1px;
     }
 
     #main-window[sizemode=normal] #browser-border-start,
     #main-window[sizemode=normal] #browser-border-end {
       display: -moz-box;
-      background-color: @toolbarShadowColor@;
+      background-color: @glassShadowColor@;
       width: 1px;
     }
 
     #main-window[sizemode=normal] #browser-bottombox {
-      border: 1px solid @toolbarShadowColor@;
+      border: 1px solid @glassShadowColor@;
       border-top-style: none;
       background-clip: padding-box;
     }
   }
 
   #appcontent:not(:-moz-lwtheme) {
     background-color: -moz-dialog;
   }

--- a/browser/themes/windows/browser.css
+++ b/browser/themes/windows/browser.css
@@ -3,16 +3,18 @@
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
 @import url("chrome://global/skin/");
 
 @namespace url("http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul");
 @namespace html url("http://www.w3.org/1999/xhtml");
 
 %include ../shared/browser.inc.css
+%filter substitution
+%define glassShadowColor hsla(209,67%,12%,0.35)
 
 :root {
   --titlebar-text-color: currentColor;
 
   --tabs-border: threedshadow;
   --tab-line-color: highlight;
 
   --toolbar-non-lwt-bgcolor: -moz-dialog;
@@ -107,17 +109,17 @@
 
 /*
  * Windows 7 draws the chrome background color as the tab background
  * instead of in the tabs toolbar.
  */
 @media (-moz-os-version: windows-win7) {
   @media (-moz-windows-default-theme) {
     :root:not(:-moz-lwtheme) {
-      --tabs-border: #4A4A4F;
+      --tabs-border: @glassShadowColor@;
     }
 
     #TabsToolbar:not(:-moz-lwtheme),
     #TabsToolbar:not(:-moz-lwtheme) toolbarbutton[disabled="true"] {
       color: hsl(240,9%,98%);
     }
 
     /* Always show full-height tab separators on tabs with borders. */