Are you syncing across your browsers via Google?

Thanks for the quick reply. I'm not sure what you mean. I'm only using Firefox. I'm not consciously syncing across anything.

• "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.