don't allow sub-folders or saved searches under smart folders inbox, r=mkmelin, 490326

mail/base/content/mailContextMenus.js

@@ -475,10 +475,10 @@ function fillFolderPaneContextMenu()
   }
   var selectedServers = folders.filter(checkIsServer);
 
-  var specialFolder;
-  if (numSelected == 1) {
-    specialFolder = getSpecialFolderString(folders[0]);
-  }
+  let specialFolder;
+  if (numSelected == 1)
+    specialFolder = haveAnyVirtualFolders ? "Virtual" :
+                                          getSpecialFolderString(folders[0]);
 
   function checkCanSubscribeToFolder(folder) {
     if (checkIsVirtualFolder(folder))

mail/base/content/mailWindowOverlay.js

@@ -103,7 +103,8 @@ function menu_new_init()
 
   const nsMsgFolderFlags = Components.interfaces.nsMsgFolderFlags;
   var isInbox = folder.isSpecialFolder(nsMsgFolderFlags.Inbox);
-  var showNew = folder.canCreateSubfolders || isInbox;
+  var showNew = folder.canCreateSubfolders ||
+                (isInbox && !(folder.flags & nsMsgFolderFlags.Virtual));
   ShowMenuItem("menu_newFolder", showNew);
   ShowMenuItem("menu_newVirtualFolder", showNew);