--- a/browser/themes/osx/browser.css
+++ b/browser/themes/osx/browser.css
@@ -980,16 +980,20 @@ toolbar .toolbarbutton-1:not([type="menu
   #nav-bar-overflow-button:hover:active:not([disabled="true"]) {
     -moz-image-region: rect(36px, 1224px, 72px, 1188px);
   }
 
   #nav-bar-overflow-button[open] {
     -moz-image-region: rect(72px, 1224px, 108px, 1188px);
   }
 
+  #nav-bar-overflow-button:-moz-locale-dir(rtl) > .toolbarbutton-icon {
+    transform: scaleX(-1);
+  }
+
   #tabview-button {
     -moz-image-region: rect(0, 1296px, 36px, 1260px);
   }
 
   #tabview-button@toolbarButtonPressed@ {
     -moz-image-region: rect(36px, 1296px, 72px, 1260px);
   }
 

--- a/browser/themes/shared/toolbarbuttons.inc.css
+++ b/browser/themes/shared/toolbarbuttons.inc.css
@@ -131,11 +131,15 @@
 #webrtc-status-button:not(@inAnyPanel@) {
   -moz-image-region: rect(0, 594px, 18px, 576px);
 }
 
 #nav-bar-overflow-button {
   -moz-image-region: rect(0, 612px, 18px, 594px);
 }
 
+#nav-bar-overflow-button:-moz-locale-dir(rtl) > .toolbarbutton-icon {
+  transform: scaleX(-1);
+}
+
 #tabview-button {
   -moz-image-region: rect(0, 648px, 18px, 630px);
 }