--- a/mail/themes/windows/mail/messenger.css
+++ b/mail/themes/windows/mail/messenger.css
@@ -449,17 +449,17 @@ toolbar[brighttext] .toolbarbutton-1 > .
   }
 
   .toolbarbutton-1[checked=true]:not(:active):hover {
     background-color: var(--toolbarbutton-checkedhover-backgroundcolor);
     transition: background-color .4s;
   }
 }
 
-menupopup:not([type="folder"]) > menuseparator {
+menulist > menupopup:not([type="folder"]) > menuseparator {
   -moz-appearance: none;
   margin-top: 3px;
   margin-bottom: 3px;
   padding-top: 0;
   padding-bottom: 0;
   border-top-color: #d7d7d7;
 }