If you need to clear your history every time you use Firefox, you can set it to happen automatically on exit so you don’t forget.
At the top of the Firefox window, click on the Firefox button (Tools menu in Windows XP) and then click Options
Select the Privacy panel.
Set Firefox will: to Use custom settings for history.
Check the box for Clear history when Firefox closes.
To specify what types of history should be cleared, click the Settings... button next to Clear history when Firefox closes.
In the Settings for Clearing History window, check the items that you want to have cleared automatically each time you quit Firefox.
After selecting the history to be cleared, click OK to close the Settings for Clearing History window.
Click OK to close the Options window