How to clear cookies/cache on MacBook Air?

Firefox has its own way of clearing website data.

Use Firefox's History menu > Clear Recent History... and choose what you want to clear.

Preferably, when you have launched Firefox using the shift key so no web page is open in the browser. If there is and you clear cookies, that site will rewrite their cookie.

Using John Galt's suggestion, I use shift+cmd+delete to pop open the clear panel (below), press return to accept it, and then cmd+Q to quit Firefox.

Settings > Privacy & Security > Cookies & Site Data > Clear Data