To restore a backup of your bookmarks, check out this article: Restore bookmarks from backup or move them to another computer.

As for what went wrong, I'm not sure, but when you selected your bookmarks, did you only select the latest 10 in the right pane??

If you haven't closed fox then try Undo (Ctrl+Z) in the Organize menu in the Library.

Otherwise you should be able to restore them from a JSON backup in the bookmarkbackups folder.

• Bookmarks > Show All Bookmarks > Import & Backup > Restore > Choose File

More info about these so called smart folders:

• "Most Visited" and "Recently Bookmarked" and "Recent Tags" are examples of so called Smart Bookmarks folders and are not real existing folders.
• Such smart folders show a list created by a query of the places.sqlite database that stores the bookmarks and the history in Firefox.
• Smart folder lists show a maximum of 10 entries by default.
• Bookmarks or history items that show up in a smart folder list are stored elsewhere in another folder and any changes made are applied to the real bookmark or history item.
• If you remove an entry then the list is shifted up and a new entry that wasn't shown previously is added to show 10 entries.
• If you add a new bookmark or visit a website then a new item is added at the top and the entry at the bottom disappears from that list.
• Items that disappear from the list are not gone, but merely do not show up anymore in that list.
• Actions like copy & paste or delete that you perform on bookmarks in such a list are done on the original bookmark.
• If you do not want a specific list then remove that query (right-click: Delete), but do not delete the content that is displayed in such a list.