To clear Cookies and Site Data from Firefox version 75.0 or higher complete the following:

1. Click the Open Menu button on the top right-side of the page. 
2. Click "Options" from the Menu (on a Mac this option will say "Preferences")
3. Click "Privacy and Security"
4. Scroll down to "Cookies and Site Data" section
5. Click Clear Data...
6. Select the checkboxes for "Cookies and Site Data" and "Cached Web Content" 
7. Click Clear
8. Click Clear Now