“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.”
Clear your cookies and the trackers still know you, because they stashed an ID in a cache or a connection. Firefox now partitions all of that by site. The cost is at most about 1.3 percent on page loads. Adtech built a business on browser features nobody meant for tracking, and browsers are only now closing them.