When clicking the Library button and then Bookmarks, a list of recently added bookmarks is displayed. Recent bookmarks are useful when a bookmark was accidentally saved in the wrong folder and cannot be found.
However, I prefer having separate Bookmarks and History buttons on the toolbar without the library button (and Firefox allows to customize the toolbar according to personal preferences), but the standalone Bookmarks button dropdown lacks recent bookmarks.
It would be useful if they were added, either in the first level menu (like History button has a "Recent history" category) or in a single entry that opens a sub-menu.