--- a/mail/themes/windows/mail/folderMenus.css
+++ b/mail/themes/windows/mail/folderMenus.css
@@ -3,16 +3,21 @@
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
 /* ===== folderMenus.css ================================================
   == Icons for menus which represent mail folder.
   ======================================================================= */
 
 @namespace url("http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul");
 
+/* Don't apply the here needless -moz-image-region to the dropmarker. */
+.menulist-dropmarker {
+  -moz-image-region: unset;
+}
+
 /* ::::: Folder icons for menus   ::::: */
 
 /* Folders */
 .folderMenuItem {
   list-style-image: url("chrome://messenger/skin/icons/folder.png");
   -moz-image-region: rect(0 16px 16px 0);
 }