--- a/toolkit/themes/shared/in-content/common.inc.css
+++ b/toolkit/themes/shared/in-content/common.inc.css
@@ -734,16 +734,20 @@ xul|richlistbox:-moz-focusring {
 xul|listheader,
 xul|treecols {
   -moz-appearance: none;
   border: none;
   border-bottom: 1px solid var(--in-content-border-color);
   padding: 0;
 }
 
+.autocomplete-tree > xul|treecols {
+  border-bottom: none !important;
+}
+
 xul|treecol:not([hideheader="true"]),
 xul|treecolpicker {
   -moz-appearance: none;
   border: none;
   background-color: var(--in-content-box-background-hover);
   color: #808080;
   padding: 5px 10px;
 }