--- a/browser/themes/osx/browser.css
+++ b/browser/themes/osx/browser.css
@@ -1148,16 +1148,17 @@ toolbarbutton.chevron > .toolbarbutton-m
 
 #TabsToolbar:not(:-moz-lwtheme) {
   color: #333;
   text-shadow: @loweredShadow@;
 }
 
 :root:-moz-any([inFullscreen], [tabsintitlebar]) #TabsToolbar:not(:-moz-lwtheme) {
   -moz-appearance: -moz-mac-vibrancy-dark;
+  background-color: #232323;
   color: hsl(240, 9%, 98%);
   text-shadow: none;
 }
 
 #tabbrowser-tabs {
   -moz-box-align: stretch;
 }