Mozilla Blog: Firefox 85 Cracks Down on Supercookies

In short, supercookies can be used in place of ordinary cookies to store user identifiers, but they are much more difficult to delete and block. This makes it nearly impossible for users to protect their privacy as they browse the web. Over the years, trackers have been found storing user identifiers as supercookies in increasingly obscure parts of the browser, including in Flash storage, ETags, and HSTS flags.