State Partitioning is an interesting privacy feature shipping with Firefox 86.
With State Partitioning, shared state such as cookies, localStorage, etc. will be partitioned (isolated) by the top-level website you’re visiting. In other words, every first party and its embedded third-party contexts will be put into a self-contained bucket.
Applies to every embedded third-party resource (regardless of whether it is a tracker or not), but some exceptions for Single Sign-On purposes have been made.
As of version 85 a similar feature landed in Chromium. Safari introduced partitioning for Cookies with its Intelligent Tracking Prevention 1.0. Can’t find any mention whether Safari also partitions other 3rd-party content, but I think they do.
☝️ Without these protections, ads follow you around the internet.