diff --git a/browser/themes/linux/customizableui/panelUIOverlay.css b/browser/themes/linux/customizableui/panelUIOverlay.css
index 4a7327b54ce..f82125007c1 100644
--- a/browser/themes/linux/customizableui/panelUIOverlay.css
+++ b/browser/themes/linux/customizableui/panelUIOverlay.css
@@ -77,6 +77,10 @@ menu[disabled="true"].subviewbutton > .menu-right {
   -moz-image-region: rect(0, 32px, 16px, 16px);
 }
 
+menu.subviewbutton > .menu-right:-moz-locale-dir(rtl) {
+  transform: scaleX(-1);
+}
+
 .subviewbutton > .toolbarbutton-icon {
   -moz-margin-end: 5px !important;
 }
diff --git a/browser/themes/windows/customizableui/panelUIOverlay.css b/browser/themes/windows/customizableui/panelUIOverlay.css
index 067644f78f5..31d428117e8 100644
--- a/browser/themes/windows/customizableui/panelUIOverlay.css
+++ b/browser/themes/windows/customizableui/panelUIOverlay.css
@@ -126,6 +126,10 @@ menu[disabled="true"].subviewbutton > .menu-right {
   -moz-image-region: rect(0, 32px, 16px, 16px);
 }
 
+menu.subviewbutton > .menu-right:-moz-locale-dir(rtl) {
+  transform: scaleX(-1);
+}
+
 %ifdef WINDOWS_AERO
 /* Win8 and beyond. */
 @media not all and (-moz-os-version: windows-vista) {