It is just the last ten items that you bookmarked.

Hello

Available from the Bookmarks menu, Recent Bookmarked contains a list of the last several bookmarks you've added. Expand the Recently Bookmarked folder to see a list of sites.

You can read more on Smart Bookmarks folders

Thank you

Note that "Recently Bookmarked" is a so called Smart Bookmark that shows the result of a place query in a list and isn't a real existing folder.

• "Most Visited" and "Recently Bookmarked" and "Recent Tags" are examples of so called Smart Bookmarks folders and are not real existing folders.
• Smart folders show a list created by a query of the places.sqlite database that stores bookmarks and history in Firefox.
• Smart folder lists show a maximum of 10 entries by default.
• Bookmarks and 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.