Because I use many bookmarks in my standard Firefox bookmarks toolbar it mainly contains folders in its main row. And if a folder is clicked it expands vertically like a windows menu. This is how it should be.
BookmarksDropDown version 2.1.0 displays a white colored bookmarks toolbar in a new design (looks not like the native Firefox bookmarks toolbar anymore). Every folder got a black colored triangle button on its left side to show that it is clickable. If you click on a folder, its content is displayed in a new horizontal toolbar. If the same folder is clicked again, then the same content is added again. So if you do this multiple times the bookmarks entries will fill the entire Firefox main window. Furthermore a single bookmark is diplayed with a radio button in front of it.
This all looks not even like an alpha version 0.0.1 to me. I think the whole concept of the new version 2.x addon isn't well thought out and needs to be redone from ground up.
The old BookmarksDropDown version 1.3.2 still works well. But its old addon format won't work in future versions of Firefox.