Most visited is a list that is generated from the browsing history based upon the visit count.<br /> If those sites have disappeared then that means that other sites have a higher visit count.<br /> That can be caused by visiting the other sites more frequently or because the Yahoo and Facebook history items were deleted.<br /> To make them reappear you will have to visit them often enough to give them a visit count that is high enough compared to other sites in that list.<br /> You can make the Visit Count column visible in the History manager to see the current visit count. *http://kb.mozillazine.org/Viewing_the_browsing_history_-_Firefox