Top sites is what is known as a "smart folder" it is actually a database query. Do not delete anything inside the query. If you have two "Top sites" folders delete either one of then or if you have it and don;t want it delete the folder, but do not delete the contents. ---- To learn more, though you answer you need is above. Most Visited -- Smart Bookmark Folders -- recently visited (History list) :https://support.mozilla.com/questions/831554 :https://support.mozilla.com/questions/858163#answer-225990 :After creating the bookmark for a "smart folder" you must, restart Firefox after which the bookmark looks like a folder and will function properly. :related: https://support.mozilla.com/kb/Smart%20Bookmarks%20folders (marked old) :related: http://kb.mozillazine.org/Viewing_the_browsing_history_-_Firefox