--- a/browser/themes/shared/toolbarbutton-icons.inc.css
+++ b/browser/themes/shared/toolbarbutton-icons.inc.css
@@ -18,17 +18,18 @@ toolbar[brighttext] {
   fill-opacity: var(--toolbarbutton-icon-fill-opacity);
 }
 
 #back-button:-moz-locale-dir(rtl) > .toolbarbutton-icon,
 #forward-button:-moz-locale-dir(rtl) > .toolbarbutton-icon,
 #reload-button:-moz-locale-dir(rtl) > .toolbarbutton-icon,
 #nav-bar-overflow-button:-moz-locale-dir(rtl) > .toolbarbutton-icon,
 #PlacesChevron:-moz-locale-dir(rtl) > .toolbarbutton-icon,
-#panic-button:-moz-locale-dir(rtl) > .toolbarbutton-icon {
+#panic-button:-moz-locale-dir(rtl) > .toolbarbutton-icon,
+#PanelUI-menu-button:-moz-locale-dir(rtl) > .toolbarbutton-badge-stack > .toolbarbutton-icon {
   transform: scaleX(-1);
 }
 
 #back-button {
   list-style-image: url("chrome://browser/skin/back.svg");
 }
 
 #forward-button {