Step 1: Open settings — Open Mozilla Firefox, click the button with three bars in the upper-right corner of the browser window and select Options from the resulting drop-down menu.
Step 2: Clear the browser cache — Click the Privacy and Security tab located on the right side of the navigation bar and head to the Cookies and Site Data section underneath. Click the button labeled Clear Data. You’ll want to make sure that the option for Cached Web Content is checked, and then press Clear. You can also check the box next to Delete cookies and site data when Firefox is closed to do so automatically when you close the browser.