Saving your entire history permanently would take a lot of space on your computer. Firefox will keep history items for a maximum of 180 days or up to 40000 history items. If you go over that limit, Firefox starts to remove old history items. Other browsers have similar limits. Also, some cleaning programs that can be downloaded will also remove old history items from your computer to save space. If there are no items in the "Old than 6 months" folder in your history, that information likely isn't saved anymore.