Web advertisers are resorting to a new technique <https://www.theregister.co.uk/2019/11/21/ublock_origin_firefox_unblockable_tracker/> to bypass restrictions on third-party cookies for tracking users’ browsing behaviour: use first-party cookies instead. A detailed example can be found in this GitHub issue <https://github.com/uBlockOrigin/uBlock-issues/issues/780>: the site www.liberation.fr includes links to f7ds.liberation.fr, which is notionally under the same domain. However, that is actually a CNAME (alias) for the third-party tracker site liberation.eulerian.net. So cookies that are supposed to be restricted to *.liberation.fr are passed to that site as well. Just to be clear, this is not a browser “bug” by any means; it’s just part of how the DNS is supposed to work. The good news is, Firefox offers an API that adblocker addons can use to detect this sort of thing going on and stop it.