Such smart folders show the result of a database query and aren't real folder. This means that you can't prevent Firefox from adding sites that match the query to such a list. If you do not want to see a list then if possible remove this bookmark via the right-click context menu or hide it in case of the newly added "Recently Bookmarked" section via the right-click context menu. ---- Some information about so called smart bookmarks folders. * https://support.mozilla.org/en-US/kb/Smart+Bookmarks+folders * "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 * most smart folder lists show a maximum of 10 entries by default * bookmark 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 * bookmarks created by clicking the star on the Navigation toolbar are created in the "Other Bookmarks" folder (formerly named "Unsorted") * bookmarks created by clicking "Bookmark This Page" (Ctrl+D) are created in the Bookmarks Menu folder * 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 * you can hide the newly added "Recently Bookmarked" section via the right-click context menu (Hide Recently Bookmarked)