Add another bookmarks Toolbar.
You can put this additional Bookmark Toolbar in the arbitrary place of the toolbox.
The number becomes 5 (original + addition 4) in total
1. Place additional bookmarks toolbar item(Bookmarks2-5) by Customize Toolbar.
2. Carry out "Edit Bookmarks Toolbar Folder" command of the context menu in added bookmarks toolbar.
3. Choose the bookmark folder which you want to display.
If you make folder list with an option, you can change folder to show in toolbar by mouse wheel on "Edit Bookmarks Toolbar Folder" of the context menu.
Similar but various functions are packed with "Incredible Bookmarks" ( https://addons.mozilla.org/en-US/firefox/addon/50722