To empty the cache in Firefox you click on the ‘hamburger menu’ to the right (the three lines in the menu bar). Here you’ll find ‘Settings’.
You’ll now see the general settings. In the menu to the left, click ‘Privacy & Security’. Scroll down to the section ‘Cookies and Site Data’, then click ‘Clear Data’ to the right of this section.
A pop-up appears where you can select which browsing data you want to delete. Select the checkboxes and click ‘Clear Now’.