@mcc First-party isolate makes it so all cookies/storage for embedded third-party resources are scoped under the domain in the URL bar. It was developed by/for Tor Browser and upstreamed in Firefox. The Mozilla folks don't much like that it exists, and have developed their own much weaker heuristic 3p tracking protections that it replaces.

The canonical example is that embedded social media widgets can never see your logged-in state on their own site. Basically, embeds for tracking don't work at all.