Close all open Firefox windows and tabs. Open a new Firefox window.
Click on "Firefox" menu and select "Preferences"
Navigate to "Privacy & Security" and find the section that reads "Cookies and Site Data."
Under Cookies and Site data, click "Clear data...". Make sure the setting for cookies and cache are bot checked, then click "clear".
Click "Clear Now". Quit Firefox and restart the browser.
To clear history, simply go to the menus at the top of Firefox, choose "History" menu and "Clear recent history." You can also remove cache/cookies from this menu, but be sure to select "All Time" for the range if you are removing cookies/cache as troubleshooting.