Third-party cookies

Third-party cookies are cookies set by a domain other than the site you're visiting: an ad server or platform embedding itself across many sites and recognizing the same browser on each. They powered cross-site retargeting, frequency capping, and view-through measurement. Safari and Firefox block them by default; Chrome, after years of deprecation plans, walked back and kept them.

#The party is who set it, not what it knows

Third-party versus first-party is about the setting context (which domain wrote the cookie), not about how sensitive the data is. The same platform can operate in both modes, which is why "we use first-party cookies" is not by itself a privacy claim. For marketers the practical story has two halves. Cross-site mechanisms are already degraded: on Safari and Firefox traffic, retargeting pools shrink and view-through attribution goes dark regardless of what Chrome does. The quieter half is the one that bites tracking stacks hardest: the same privacy push shortened first-party cookie lifetimes (Safari's ITP caps script-set cookies at seven days), which silently truncates attribution windows on every stack, consent banner or not.

In practice: Buron's measurement is built on the durable side of this shift: a first-party pixel and warehouse-side matching rather than cross-site cookies.

What still works after third-party cookies, including the living cookie-lifetime table, is the full page: Tracking after third-party cookies: what actually still works. The lifetime caps get their own entry at Cookie lifetime / ITP, and the territory routes from Conversion tracking & signal quality.