--- a/browser/themes/shared/toolbarbutton-icons.inc.css
+++ b/browser/themes/shared/toolbarbutton-icons.inc.css
@@ -398,16 +398,20 @@ toolbar:not([brighttext]) #bookmarks-men
   animation-name: overflow-animation-rtl;
 }
 
 #nav-bar-overflow-button[animate][fade] > .toolbarbutton-animatable-box > .toolbarbutton-animatable-image {
   animation-name: overflow-fade;
   animation-timing-function: ease-out;
   animation-duration: 730ms;
 }
+
+#nav-bar-overflow-button[animate][fade]:-moz-locale-dir(rtl) > .toolbarbutton-animatable-box > .toolbarbutton-animatable-image {
+  transform: scaleX(-1);
+}
 %endif
 
 #email-link-button@attributeSelectorForToolbar@ {
   list-style-image: url("chrome://browser/skin/mail.svg");
 }
 
 #sidebar-button@attributeSelectorForToolbar@ {
   list-style-image: url("chrome://browser/skin/sidebars-right.svg");