diff --git a/browser/themes/windows/browser-aero.css b/browser/themes/windows/browser-aero.css
index ccfdef5afc2d..fa1b363b7afe 100644
--- a/browser/themes/windows/browser-aero.css
+++ b/browser/themes/windows/browser-aero.css
@@ -326,6 +326,11 @@
     border-top-right-radius: 2.5px;
   }
 
+  /* Set to full fill-opacity to improve visibility of toolbar buttons on aero glass. */
+  #TabsToolbar:not(:-moz-lwtheme) {
+    --toolbarbutton-icon-fill-opacity: 1;
+  }
+
   #toolbar-menubar:not(:-moz-lwtheme) {
     text-shadow: 0 0 .5em white, 0 0 .5em white, 0 1px 0 rgba(255,255,255,.4);
   }
diff --git a/browser/themes/windows/compacttheme.css b/browser/themes/windows/compacttheme.css
index 53a866a4e037..e72e3fa36be2 100644
--- a/browser/themes/windows/compacttheme.css
+++ b/browser/themes/windows/compacttheme.css
@@ -46,6 +46,11 @@
 }
 
 @media (-moz-windows-glass) {
+  /* Set to full fill-opacity to improve visibility of toolbar buttons on aero glass. */
+  #TabsToolbar {
+    --toolbarbutton-icon-fill-opacity: 1;
+  }
+
   /* Make the menubar text readable on aero glass (copied from browser-aero.css). */
   #toolbar-menubar {
     text-shadow: 0 0 .5em white, 0 0 .5em white, 0 1px 0 rgba(255,255,255,.4);