--- a/browser/themes/linux/browser.css
+++ b/browser/themes/linux/browser.css
@@ -25,52 +25,41 @@
   --toolbarbutton-vertical-text-padding: calc(var(--toolbarbutton-inner-padding) - 1px);
 
   --panel-separator-color: ThreeDShadow;
   --arrowpanel-dimmed: hsla(0,0%,80%,.3);
   --arrowpanel-dimmed-further: hsla(0,0%,80%,.45);
   --arrowpanel-dimmed-even-further: hsla(0,0%,80%,.8);
 
   --urlbar-separator-color: ThreeDShadow;
+
+  --toolbox-border-bottom-color: ThreeDShadow;
 }
 
 :root:-moz-lwtheme {
   --toolbar-bgcolor: rgba(255,255,255,.4);
   --toolbar-bgimage: none;
+
+  --toolbox-border-bottom-color: rgba(0,0,0,.3);
 }
 
 #menubar-items {
   -moz-box-orient: vertical; /* for flex hack */
 }
 
 #main-menubar {
   -moz-box-flex: 1; /* make menu items expand to fill toolbar height */
 }
 
 #navigator-toolbox {
   -moz-appearance: none;
   background-color: transparent;
   border-top: none;
 }
 
-#navigator-toolbox::after {
-  content: "";
-  display: -moz-box;
-  -moz-box-ordinal-group: 101; /* tabs toolbar is 100 */
-  border-bottom: 1px solid ThreeDShadow;
-}
-
-#navigator-toolbox:-moz-lwtheme::after {
-  border-bottom-color: rgba(0,0,0,.3);
-}
-
-:root[customizing] #navigator-toolbox::after {
-  display: none;
-}
-
 #navigator-toolbox > toolbar:not(#toolbar-menubar):not(#TabsToolbar) {
   background-color: var(--toolbar-bgcolor);
   background-image: var(--toolbar-bgimage);
   -moz-appearance: none;
   border-style: none;
 }
 
 #TabsToolbar:not([collapsed="true"]) + #nav-bar {

--- a/browser/themes/osx/browser.css
+++ b/browser/themes/osx/browser.css
@@ -26,44 +26,25 @@
   --toolbarbutton-border-radius: 4px;
 
   --panel-separator-color: hsla(210,4%,10%,.14);
   --arrowpanel-dimmed: hsla(210,4%,10%,.07);
   --arrowpanel-dimmed-further: hsla(210,4%,10%,.12);
   --arrowpanel-dimmed-even-further: hsla(210,4%,10%,.17);
 
   --urlbar-separator-color: hsla(0,0%,16%,.2);
+
+  --toolbox-border-bottom-color: #e1e1e2;
 }
 
 :root:-moz-lwtheme {
   --toolbar-bgcolor: rgba(255,255,255,.4);
   --toolbar-bgimage: none;
-}
 
-#navigator-toolbox::after {
-  -moz-box-ordinal-group: 101; /* tabs toolbar is 100 */
-  content: "";
-  display: -moz-box;
-  border-top: 1px solid hsla(0,0%,100%,.15);
-  border-bottom: 1px solid hsla(0,0%,0%,.15);
-  margin-top: -2px;
-  position: relative;
-  z-index: 2; /* navbar is at 1 */
-}
-
-@media (-moz-mac-yosemite-theme) {
-  #navigator-toolbox:-moz-window-inactive::after {
-    border-top-style: none;
-    border-bottom-color: hsla(0,0%,0%,.1);
-    margin-top: -1px;
-  }
-}
-
-:root[customizing] #navigator-toolbox::after {
-  display: none;
+  --toolbox-border-bottom-color: rgba(0,0,0,.3);
 }
 
 #navigator-toolbox toolbarbutton:-moz-lwtheme {
   color: inherit;
   text-shadow: inherit;
 }
 
 #main-window {
@@ -129,21 +110,16 @@
   border-top: 1px solid rgba(0,0,0,0.65);
 }
 
 #navigator-toolbox > toolbar:not(#TabsToolbar) {
   -moz-appearance: none;
   background: var(--toolbar-bgcolor);
 }
 
-#navigator-toolbox > toolbar:not(#toolbar-menubar):not(#TabsToolbar):not(#nav-bar) {
-  /* add extra padding to compensate the bottom border */
-  padding-bottom: 2px;
-}
-
 /* Draw the bottom border of the tabs toolbar when it's not using
    -moz-appearance: toolbar. */
 #main-window:-moz-any([sizemode="fullscreen"],[customize-entered]) #TabsToolbar:not([collapsed="true"]) + #nav-bar,
 #main-window:not([tabsintitlebar]) #TabsToolbar:not([collapsed="true"]) + #nav-bar,
 #TabsToolbar:not([collapsed="true"]) + #nav-bar:-moz-lwtheme {
   border-top: 1px solid var(--tabs-border);
   background-clip: padding-box;
   /* Position the toolbar above the bottom of background tabs */

--- a/browser/themes/shared/browser.inc.css
+++ b/browser/themes/shared/browser.inc.css
@@ -11,16 +11,28 @@
 %else
 :root[tabsintitlebar][sizemode=normal] #TabsToolbar
 %endif
 {
   padding-inline-start: 40px;
 }
 %endif
 
+/* Toolbar / content area border */
+
+#navigator-toolbox::after {
+  content: "";
+  display: -moz-box;
+  border-bottom: 1px solid var(--toolbox-border-bottom-color);
+}
+
+:root[customizing] #navigator-toolbox::after {
+  display: 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;
   padding: 0 4px 1px;
 }

--- a/browser/themes/shared/compacttheme.inc.css
+++ b/browser/themes/shared/compacttheme.inc.css
@@ -16,17 +16,17 @@
   --tab-line-color: highlight;
 }
 
 :root:-moz-lwtheme-brighttext {
   /* Chrome */
   --chrome-background-color: hsl(240, 5%, 5%);
   --chrome-color: rgb(249, 249, 250);
   --chrome-secondary-background-color: hsl(240, 1%, 20%);
-  --chrome-navigator-toolbox-separator-color: hsla(240, 5%, 5%, .1);
+  --toolbox-border-bottom-color: hsla(240, 5%, 5%, .1);
   --chrome-nav-bar-separator-color: rgba(0,0,0,.2);
   --chrome-nav-buttons-background: hsla(240, 5%, 5%, .1);
   --chrome-nav-buttons-hover-background: hsla(240, 5%, 5%, .15);
   --chrome-nav-bar-controls-border-color: hsla(240, 5%, 5%, .3);
   --chrome-selection-color: #fff;
   --chrome-selection-background-color: #5675B9;
 
   /* Url and search bars */
@@ -35,17 +35,17 @@
 }
 
 :root:-moz-lwtheme-darktext {
   --url-and-searchbar-background-color: #fff;
 
   --chrome-background-color: #E3E4E6;
   --chrome-color: #18191a;
   --chrome-secondary-background-color: #f5f6f7;
-  --chrome-navigator-toolbox-separator-color: #cccccc;
+  --toolbox-border-bottom-color: #cccccc;
   --chrome-nav-bar-separator-color: #B6B6B8;
   --chrome-nav-buttons-background: #ffffff; /* --theme-body-background */
   --chrome-nav-buttons-hover-background: #DADBDB;
   --chrome-nav-bar-controls-border-color: #ccc;
   --chrome-selection-color: #f5f7fa;
   --chrome-selection-background-color: #4c9ed9;
 }
 
@@ -63,20 +63,16 @@ toolbar[brighttext] .toolbarbutton-1 {
 /* Change the base colors for the browser chrome */
 
 #TabsToolbar,
 #browser-panel {
   background: var(--chrome-background-color);
   color: var(--chrome-color);
 }
 
-#navigator-toolbox:-moz-lwtheme::after {
-  border-bottom-color: var(--chrome-navigator-toolbox-separator-color);
-}
-
 #navigator-toolbox > toolbar:not(#TabsToolbar):not(#toolbar-menubar),
 .browserContainer > findbar,
 #browser-bottombox {
   background-color: var(--chrome-secondary-background-color) !important;
   background-image: none !important;
   color: var(--chrome-color);
 }
 

--- a/browser/themes/windows/browser-aero.css
+++ b/browser/themes/windows/browser-aero.css
@@ -289,26 +289,24 @@
   :root[darkwindowframe="true"]:not(:-moz-lwtheme):not(:-moz-window-inactive) {
     --titlebar-text-color: white;
   }
 
   /* 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: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 > toolbar:not(#toolbar-menubar):not(#TabsToolbar) {
+    #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@;
       background-clip: padding-box;
     }
 
-    #main-window:not([customizing])[sizemode=normal] #navigator-toolbox:not(:-moz-lwtheme)::after,
-    #main-window[customizing] #navigator-toolbox::after {
+    #main-window[sizemode=normal] #navigator-toolbox:not(:-moz-lwtheme)::after {
       box-shadow: 1px 0 0 @toolbarShadowColor@, -1px 0 0 @toolbarShadowColor@;
       margin-left: 1px;
       margin-right: 1px;
     }
 
     #main-window[sizemode=normal] #browser-border-start,
     #main-window[sizemode=normal] #browser-border-end {
       display: -moz-box;

--- a/browser/themes/windows/browser.css
+++ b/browser/themes/windows/browser.css
@@ -25,35 +25,41 @@
   --toolbarbutton-border-radius: 2px;
 
   --panel-separator-color: ThreeDLightShadow;
   --arrowpanel-dimmed: hsla(0,0%,80%,.3);
   --arrowpanel-dimmed-further: hsla(0,0%,80%,.45);
   --arrowpanel-dimmed-even-further: hsla(0,0%,80%,.8);
 
   --urlbar-separator-color: ThreeDLightShadow;
+
+  --toolbox-border-bottom-color: ThreeDShadow;
 }
 
 @media (-moz-windows-default-theme) {
   :root {
     --tabs-border: rgba(0,0,0,.3);
 
     --toolbar-non-lwt-bgcolor: #f9f9fa;
     --toolbar-non-lwt-textcolor: #0c0c0d;
     --toolbar-non-lwt-bgimage: none;
 
     --panel-separator-color: hsla(210,4%,10%,.14);
+
+    --toolbox-border-bottom-color: #e1e1e2;
   }
 }
 
 :root:-moz-lwtheme {
   --tabs-border: rgba(0,0,0,.3);
 
   --toolbar-bgcolor: rgba(255,255,255,.4);
   --toolbar-bgimage: none;
+
+  --toolbox-border-bottom-color: rgba(0,0,0,.3);
 }
 
 #menubar-items {
   -moz-box-orient: vertical; /* for flex hack */
 }
 
 #main-menubar {
   -moz-box-flex: 1; /* make menu items expand to fill toolbar height */
@@ -65,40 +71,16 @@
   display: none;
 }
 
 #navigator-toolbox,
 #navigator-toolbox > toolbar {
   -moz-appearance: none;
 }
 
-#navigator-toolbox::after {
-  content: "";
-  display: -moz-box;
-  -moz-box-ordinal-group: 101; /* tabs toolbar is 100 */
-  border-bottom: 1px solid ThreeDShadow;
-}
-
-@media (-moz-windows-default-theme) {
-  @media (-moz-os-version: windows-win8),
-         (-moz-os-version: windows-win10) {
-    #navigator-toolbox::after {
-      border-bottom-color: #c2c2c2;
-    }
-  }
-}
-
-#navigator-toolbox:-moz-lwtheme::after {
-  border-bottom-color: rgba(0,0,0,.3);
-}
-
-:root[customizing] #navigator-toolbox::after {
-  display: none;
-}
-
 #navigator-toolbox > toolbar:not(#toolbar-menubar):not(#TabsToolbar) {
   background-color: var(--toolbar-bgcolor);
   background-image: var(--toolbar-bgimage);
   background-clip: padding-box;
 }
 
 #toolbar-menubar,
 #TabsToolbar {

--- a/browser/themes/windows/compacttheme.css
+++ b/browser/themes/windows/compacttheme.css
@@ -123,26 +123,16 @@
   border-right: none !important;
 }
 
 /* Disable dragging like in the default theme: */
 #main-window[tabsintitlebar] #navigator-toolbox > toolbar:not(#toolbar-menubar):not(#TabsToolbar):-moz-lwtheme {
   -moz-window-dragging: no-drag;
 }
 
-@media (-moz-os-version: windows-win7),
-       (-moz-os-version: windows-win8) {
-  /* And then we add them back on toolbars so that they don't look borderless: */
-  #main-window:not([customizing])[sizemode=normal] #navigator-toolbox::after,
-  #main-window:not([customizing])[sizemode=normal] #navigator-toolbox > toolbar:not(#toolbar-menubar):not(#TabsToolbar) {
-    border-left: 1px solid hsla(209,67%,12%,0.35);
-    border-right: 1px solid hsla(209,67%,12%,0.35);
-  }
-}
-
 @media (-moz-os-version: windows-win10) {
   .titlebar-button:-moz-lwtheme {
     -moz-context-properties: stroke;
     stroke: currentColor;
   }
   #titlebar-min:-moz-lwtheme {
     list-style-image: url(chrome://browser/skin/window-controls/minimize.svg);
   }

--- a/testing/web-platform/meta/scroll-anchoring/anchoring-with-bounds-clamping.html.ini
+++ b/testing/web-platform/meta/scroll-anchoring/anchoring-with-bounds-clamping.html.ini
@@ -1,6 +1,5 @@
 [anchoring-with-bounds-clamping.html]
   type: testharness
   [Anchoring combined with scroll bounds clamping in the document.]
-    expected:
-      if os != "mac": FAIL
+    expected: FAIL