The data traces left during surfing should be deleted, especially on PCs used by more than one person. Firefox gets rid of such traces as cookies, the cache, the list of sites visited, etc. under the menu item "Tools/Clear Private Data...".
If you want to retain the cookies and passwords for individual sites, choose the cookies and passwords that you want to have deleted under the Privacy tab of the Options dialogue box accessed via "Tools/Options". In the subtab "Cookies", press the button "View Cookies":
This button leads you to a dialogue box that display all cookies so that you can delete the ones you no longer want.