How are you checking to see if it was cleared? Some people expect the location bar drop-down list to be empty, now knowing that it lists bookmarks as well.

Reply to Chris_Ilias I have checked History>Show All History and it shows my history all the way back to April.

If you close Firefox, then start Firefox in Safe Mode, does the problem still occur? For instructions on starting in Safe Mode, see Diagnose Firefox issues using Troubleshoot Mode.

See also http://kb.mozillazine.org/Locked_or_damaged_places.sqlite

Reply to cor-el Thanks I tried deleting all the places.sqlite files and my history was cleared thanks a lot for your help. Thanks Chris_Ilias too:)