Use the shortcut Ctrl+Shift+Del in the browser and the 'Clear browsing data' settings will appear to enable you to choose what you want to clear.
To clear cookies and cache, paste the following path into Firefox's address field:
Under "Cookies and Site Data"
Click "Clear Data..."
Select only what is to be cleared.
Click "Remove Selected"
Click "Remove All"
To completely reset Firefox, paste the following path into its address field:
Click "Refresh Firefox..."