--- a/calendar/base/themes/windows/today-pane.css
+++ b/calendar/base/themes/windows/today-pane.css
@@ -92,18 +92,18 @@
   sidebarheader > spacer {
     min-height: 25px;
   }
 }
 
 @media all and (-moz-windows-compositor) {
   @media not all and (-moz-os-version: windows-win10) {
     #messengerWindow[sizemode=normal] #today-pane-panel {
-      border-inline-end: 1px solid rgba(10%, 10%, 10%, .4);
-      border-bottom: 1px solid rgba(10%, 10%, 10%, .4);
+      border-inline-end: 1px solid hsla(240, 5%, 5%, .3);
+      border-bottom: 1px solid hsla(240, 5%, 5%, .3);
       background-clip: padding-box;
     }
   }
 
   .today-pane-cycler {
     margin-top: -1px;
   }
 }

--- a/mail/themes/windows/mail/activity/activity.css
+++ b/mail/themes/windows/mail/activity/activity.css
@@ -21,17 +21,17 @@
 
 @media (-moz-windows-glass) {
   #activityManager {
     -moz-appearance: -moz-win-glass;
     background: transparent;
   }
 
   .activityview {
-    border: 1px solid rgba(10%, 10%, 10%, .4);
+    border: 1px solid hsla(240, 5%, 5%, .3);
     background-clip: padding-box
   }
 
   #clearListButton {
     margin-inline-start: 0;
     margin-bottom: 0;
   }
 }

--- a/mail/themes/windows/mail/mailWindow1.css
+++ b/mail/themes/windows/mail/mailWindow1.css
@@ -693,35 +693,35 @@ treechildren::-moz-tree-cell-text(imapde
     background-color: transparent;
   }
 }
 
 @media (-moz-os-version: windows-win7) and (-moz-windows-default-theme),
        (-moz-os-version: windows-win8) and (-moz-windows-default-theme) {
   @media (-moz-windows-compositor) {
     :root:not(:-moz-lwtheme) {
-      --tabs-border: rgba(10%, 10%, 10%, .4);
+      --tabs-border: var(--glassShadowColor);
     }
 
     #messengerWindow[sizemode=normal] #tabpanelcontainer {
-      border: 1px solid rgba(10%, 10%, 10%, .4);
+      border: 1px solid var(--glassShadowColor);
       border-top: none;
       background-clip: padding-box;
     }
 
     #messengerWindow[sizemode=normal] #mail-toolbar-menubar2 {
-      border-right: 1px solid rgba(10%, 10%, 10%, .4);
-      border-left: 1px solid rgba(10%, 10%, 10%, .4);
+      border-right: 1px solid var(--glassShadowColor);
+      border-left: 1px solid var(--glassShadowColor);
       background-clip: padding-box;
     }
 
     #messengerWindow[sizemode=normal] statusbar {
       -moz-appearance: none;
       margin-top: -1px;
-      border: 1px solid rgba(10%, 10%, 10%, .4);
+      border: 1px solid var(--glassShadowColor);
       border-top-color: threedshadow;
       border-radius: 1px 1px 0 0;
       background-clip: padding-box;
     }
   }
 }
 
 @media (-moz-os-version: windows-win8) and (-moz-windows-default-theme),

--- a/mail/themes/windows/mail/messenger.css
+++ b/mail/themes/windows/mail/messenger.css
@@ -10,16 +10,17 @@
 @import url("chrome://messenger/content/messenger.css");
 @import url("chrome://messenger/skin/shared/messenger.css");
 
 :root {
   --tab-min-height: 30px;
   --tabs-border: threedshadow;
   --tab-line-color: highlight;
 
+  --glassShadowColor: hsla(240, 5%, 5%, .3);
   --toolbar-non-lwt-bgcolor: -moz-dialog;
   --toolbar-non-lwt-textcolor: -moz-dialogtext;
   --toolbar-non-lwt-bgimage: linear-gradient(rgba(255,255,255,.15), rgba(255,255,255,.15));
   --toolbar-bgcolor: var(--toolbar-non-lwt-bgcolor);
   --toolbar-bgimage: var(--toolbar-non-lwt-bgimage);
   --toolbarbutton-border-radius: 2px;
   --toolbarbutton-icon-fill-opacity: .85;
   --toolbarbutton-hover-background: rgba(0, 0, 0, .1);

--- a/mail/themes/windows/mail/tabmail.css
+++ b/mail/themes/windows/mail/tabmail.css
@@ -147,22 +147,33 @@ tabpanels {
   @media (-moz-windows-glass: 0) {
     :root[tabsintitlebar][sizemode=normal] #mail-toolbar-menubar2[autohide=true][inactive] ~
       #tabs-toolbar {
       margin-top: 6px;
     }
   }
 }
 
+@media (-moz-windows-glass) {
+  /* draw always a top border with Glass */
+  #tabs-toolbar {
+    --tabs-top-border-width: 1px;
+  }
+
+  .tab-background {
+    border-top-style: solid;
+  }
+}
+
 @media (-moz-windows-glass),
        (-moz-os-version: windows-win8) and (-moz-windows-default-theme) {
   :root[sizemode=normal] .tabmail-tab[first-tab][visuallyselected]:before {
     content: '';
     display: -moz-box;
-    --tabs-border: rgba(10%, 10%, 10%, .4);
+    --tabs-border: var(--glassShadowColor);
   }
 }
 
 @media (-moz-os-version: windows-win8) and (-moz-windows-default-theme) {
   #messengerWindow[darkwindowframe="true"]
   #tabs-toolbar:not(:-moz-lwtheme):not(:-moz-window-inactive),
   #messengerWindow[darkwindowframe="true"]
   .tabmail-tab:not([selected="true"]):not(:-moz-lwtheme):not(:-moz-window-inactive) {