To delete cookies for any single website:
Click the menu button [that looks like three stacked horizontal lines] and select Preferences.
Select the Privacy & Security panel and go to the Cookies and Site Data section.
Click the Manage Data… button. The Manage Cookies and Site Data dialog will appear.
In the Search websites field, type the name of the site [metafilter.com] whose cookies you want to remove. Items that match your search will be displayed.
To remove all cookies and storage data for the website, click Remove All Shown.
To remove selected items, select an entry and click Remove Selected.
Click Save Changes.
In the Removing Cookies and Site Data confirmation dialog that opens, click OK.
Close the about:preferences page.