--- a/browser/themes/shared/tabs.inc.css
+++ b/browser/themes/shared/tabs.inc.css
@@ -515,16 +515,20 @@ tabbrowser {
   right: 0;
 }
 
 #tabbrowser-tabs[positionpinnedtabs] > .tabbrowser-tab[pinned]:-moz-locale-dir(rtl)::after {
   right: unset;
   left: 0;
 }
 
+#tabbrowser-tabs[positionpinnedtabs] > .tabbrowser-tab[pinned] > .tab-stack {
+  border-inline-end: 1px solid transparent;
+}
+
 .tabbrowser-tab:-moz-any([image], [pinned]) > .tab-stack > .tab-content[attention]:not([selected="true"]),
 .tabbrowser-tab > .tab-stack > .tab-content[pinned][titlechanged]:not([selected="true"]) {
   background-image: url(chrome://browser/skin/tabbrowser/indicator-tab-attention.svg);
   background-position: center bottom calc(-4px + var(--tab-toolbar-navbar-overlap));
   background-repeat: no-repeat;
 }
 
 .tabbrowser-tab[image] > .tab-stack > .tab-content[attention]:not([pinned]):not([selected="true"]) {