--- a/browser/themes/shared/tabs.inc.css
+++ b/browser/themes/shared/tabs.inc.css
@@ -300,16 +300,20 @@
 #toolbar-menubar:not([autohide=true]) ~ #TabsToolbar > #tabbrowser-tabs > .tabbrowser-tab > .tab-stack > .tab-background {
   border-top-style: solid;
 }
 
 .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)));
+  border-image-slice: 1;
   background-color: var(--toolbar-bgcolor);
   background-image: var(--toolbar-bgimage);
   background-repeat: repeat-x;
 }
 
 :root:not([sizemode=normal]) .tabbrowser-tab[first-visible-tab] > .tab-stack > .tab-background[selected=true] {
   border-inline-start-style: none;
 }