--- a/browser/themes/shared/tabs.inc.css
+++ b/browser/themes/shared/tabs.inc.css
@@ -303,19 +303,21 @@
 
 /* Selected tab */
 
 .tab-background {
   border: 1px none transparent;
   background-clip: padding-box;
 }
 
+%ifdef MENUBAR_CAN_AUTOHIDE
 #toolbar-menubar:not([autohide=true]) ~ #TabsToolbar > #tabbrowser-tabs > .tabbrowser-tab > .tab-stack > .tab-background {
   border-top-style: solid;
 }
+%endif
 
 .tab-background[selected=true] {
   border-left-style: solid;
   border-right-style: solid;
   border-color: var(--tabs-border);
   border-image: linear-gradient(var(--tabs-border),
                                 var(--tabs-border) calc(100% - 1px - var(--tab-toolbar-navbar-overlap)),
                                 transparent calc(100% - 1px - var(--tab-toolbar-navbar-overlap)));