--- a/mail/themes/qute/mail/mailWindow1-aero.css
+++ b/mail/themes/qute/mail/mailWindow1-aero.css
@@ -21,16 +21,22 @@
 treecol.threadColumnHeader {
   -moz-padding-end: 2px;
 }
 
 treechildren::-moz-tree-image(news, threadCol, watch) {
   -moz-margin-start: -2px;
 }
 
+#mailContent[layout="wide"] #threadPaneBox,
+#mailContent[layout="wide"] #folderPaneBox {
+  margin-bottom: 0;
+  border-bottom: 0;
+}
+
 #folderpane_splitter,
 #threadpane-splitter:not([orient="vertical"]) {
   border: none;
   border-left: 1px solid #A9B7C9;
   min-width: 0;
   width: 5px;
   background-color: transparent;
   margin-top: 0;
@@ -54,16 +60,20 @@ treechildren::-moz-tree-image(news, thre
 #folderpane_splitter[state="collapsed"]:hover:-moz-locale-dir(ltr) {
   border-left: 4px solid highlight;
 }
 
 #folderpane_splitter[state="collapsed"]:hover:-moz-locale-dir(rtl) {
   border-right: 4px solid highlight;
 }
 
+#mailContent[layout="wide"] #folderpane_splitter {
+  margin-bottom: 0;
+}
+
 #threadpane-splitter,
 #attachment-splitter {
   border: none;
   border-bottom: 1px solid #A9B7C9;
   min-height: 0;
   height: 5px;
   background-color: transparent;
   margin-top: -5px;