1 - Open Firefox.

2 - Click the menu button (three horizontal lines) in the upper-right corner of the browser.

3 - Select "Settings" from the drop-down menu.

4 - In the Settings window, click on Privacy & Security on the left-hand sidebar.

5 - Scroll down to the Cookies and Site Data section.

6 - Click on "Clear Data...".

7 - In the pop-up window, make sure "Cookies and Site Data" is checked (you can also select "Cached Web Content" if you want to clear the cache as well).

8 - Click "Clear".