Go to about:preferences#privacy, scroll down to "Cookies and Site Data" then click "Clear Data." That opens a box where you can select what you want to clear and from what time-frame.

For quick acces, you can also put a History icon on your menu bar and clear your history data from there. Right-click on an empty space in your menu bar, then Left-click on the "Customize Toobar...". Next, Right-click (and keep your finger held down) on the Clock Icon, then drag it onto the menu bar.

How do I clear cookies/cache?

See https://support.mozilla.org/en-US/kb/how-clear-firefox-cache and https://support.mozilla.org/en-US/kb/clear-cookies-and-site-data-firefox