* "Most Visited" and "Recently Bookmarked" are examples of the 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. * The bookmarks that show up in such smart folder listing are stored elsewhere in another folder and any changes made are applied to the real bookmark or history item. * Such lists show a maximum of 10 entries by default and 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. * The items that are removed from the list are not gone, but merely do not show up anymore in that list. * Any 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.