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Consider a third-party cookie login flow where you go to one site to enter your password different from the site you are logging into. The password site sets a

Consider a third-party cookie login flow where you go to one site to enter your password different from the site you are logging into. The password site sets a cookie which the site you are logging into will use to determine if your password was correct or not. If I follow TPC, this pattern would break because that cookie is now isolated from the site which wants it. This used to be somewhat common. It is not anymore recently.

andrewaylett 2y ago
It was never great, but Safari's anti-tracking was probably what killed it, rather than anything Firefox does. Safari was certainly the driver of significant similar changes in projects I've worked with. My recollection is that Firefox has exclusions for requests to third parties where that third-party just redirected the user to the site, in order to allow the login flow you wrote about. Don't ask me for a cite, though. Safari, on the other hand, will (or at one point would) block requests to domains if it's seen more than one domain making third-party requests to that domain and it's not been visited as a first-party.