If clearing cookies didn't help then it is possible that the cookies.sqlite file that stores the cookies got corrupted.

Rename (or delete) cookies.sqlite (cookies.sqlite.old) and delete other present cookies files like cookies.sqlite-journal in the Firefox profile folder in case the file cookies.sqlite got corrupted. http://kb.mozillazine.org/Cookies https://support.mozilla.org/kb/Deleting+cookies

Make sure you do not run Firefox in permanent Private Browsing mode (Always use Private Browsing mode; Never Remember History).

• https://support.mozilla.org/en-US/kb/private-browsing-use-firefox-without-history

• Options/Preferences -> Privacy & Security
  Firefox will: "Use custom settings for history"
• remove checkmark: [ ] "Always use Private Browsing mode"

Thank you both for your replies; unfortunately, all cookies remain.

Is this about cookies or about local storage (cookies show a number under the Cookies column in manage data)?

Thank you, cor-el, for your question. All cookies, as well as storage-items, remain.

Note that you probably need to reload the about:preferences page to get an updated view of the Cookie Manager window. Closing and reopening this window doesn't seem to work.

Thank you, cor-el, for the workaround! I hope this small bug can now be fixed – at least at some point in the future.