Always show the Bookmarks Root and the Bookmarks Toolbar folder in the drop down menu. Their position in the dropdown are still determined by their LastModifiedDate, it's only when they don't make the short list that they are put at the bottom of the drop down.

browser/components/bookmarks/src/nsBookmarksService.cpp

@@ -3458,7 +3458,7 @@ nsBookmarksService::GetLastModifiedFolders(nsISimpleEnumerator **aResult)
             folderArray.AppendObject(element);
     }
 
-    // sorting the array containing all the folders
+    // sort the array containing all the folders
     folderArray.Sort(CompareLastModifiedFolders, NS_STATIC_CAST(void*, mInner));
 
     // only keep the first elements
@@ -3466,6 +3466,21 @@ nsBookmarksService::GetLastModifiedFolders(nsISimpleEnumerator **aResult)
     for (index = folderArray.Count()-1; index >= MAX_LAST_MODIFIED_FOLDERS; index--)
         folderArray.RemoveObjectAt(index);
 
+    // always show the bookmarks root
+    if (folderArray.IndexOfObject(kNC_BookmarksRoot) < 0)
+        folderArray.ReplaceObjectAt(kNC_BookmarksRoot, MAX_LAST_MODIFIED_FOLDERS-1);
+
+    // always show the bookmarks toolbar folder
+    nsCOMPtr<nsIRDFResource> btfResource;
+    rv = GetBookmarksToolbarFolder(getter_AddRefs(btfResource));
+    if (NS_SUCCEEDED(rv) && folderArray.IndexOfObject(btfResource) < 0) {
+        if (folderArray.ObjectAt(MAX_LAST_MODIFIED_FOLDERS-1) == kNC_BookmarksRoot)
+            index = MAX_LAST_MODIFIED_FOLDERS - 2;
+        else
+            index = MAX_LAST_MODIFIED_FOLDERS - 1;
+        folderArray.ReplaceObjectAt(btfResource, index);
+    }
+
     return NS_NewArrayEnumerator(aResult, folderArray);
 }