Click the Library button (looks like books), 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 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). We recommend at least the last day. 

Click the Clear Now button. The window will close and the items you've selected will be cleared from your history.