Bug 1418821 - Conversations with unread messages should be bold, r=nhnt11.

mail/components/im/content/imconv.xml

@@ -136,6 +136,7 @@
           let unreadCount = this.conv.unreadIncomingMessageCount;
           let directedMessages = unreadCount;
           if (unreadCount) {
+            this.setAttribute("unread", "true");
             if (this.conv.isChat) {
               directedMessages = this.conv.unreadTargetedMessageCount;
               if (directedMessages)