Click the Library button , click History and then click Clear Recent History….
Select how much history you want to clear:
Click the drop-down menu next to Time range to clear to choose how much of your history Firefox will clear (the last hour, the last two hours, the last four hours, the last day or everything).
Use the check boxes to select what information you want to clear from your history.
Click the Clear Now button. The window will close and the items you’ve selected will be cleared from your history.