This is the same problem as another user reported with Firefox 55 two months ago, which was never responded to: https://support.mozilla.org/en-US/questions/1174598 I am making a new post with more detail in the hope of getting a response, because this is not a minor problem. My cookie settings in Firefox have always been set to never allow third-party cookies. When clicking on "Show Cookies" in the privacy menu, that setting appears to be working. However, when running the most recent version of CCleaner today (5.36.6278 x64), there were hundreds of third-party cookies listed for Firefox, placed by every website I have visited since I last used CCleaner many months ago. I cleaned all the cookies out with CCleaner, then started Firefox again, visited a couple of news sites, then closed Firefox and re-ran CCleaner. Again, CCleaner showed a few third-party cookies present even though Firefox is still set to never allow them and they are not listed in "Show Cookies." So Firefox's cookie handling appears to be broken for a long time now. If this is just some bug with CCleaner then that would be a relief, but that doesn't seem likely. I hope this would be a priority fix. I am using Firefox 56.0.2 x64 on Windows 7. I do not have any cookie-related extensions enabled, although I do have the Ghostery extension running. EDIT: To confirm that no extension was causing this problem, I restarted Firefox in safe mode with everything disabled (but with third-party cookies still set to "Never Allow"), and then visited a few websites - theguardian.com, youtube.com, etc. When I re-ran CCleaner, there were dozens of third-party cookies detected, so it's not a Firefox extension that could be causing this problem.