From version 50 we have the only option to rise bookmark subfolders in the list combined with plain bookmarks - right click on parent folder and Sort alphabetically. But situation before, when new bookmarks were added in the bottom (or top - which is not important) with subfolders always upper then all plain bookmarks - it seems to me more suitable and logical, and it should be as default. Right mouse menu should be
-> By time added
and subfolder of course should be always upper plain items, as when we sort files in folders by name or timestamp, and it not involve subfolders placing.