"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 3. 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.