It's this.

Firefox' total cookie protection does not block third party cookies, it isolates them in separate jars for each website....

Total Cookie Protection works by creating a separate “cookie jar” for each website you visit. Instead of allowing trackers to link up your behavior on multiple sites, they just get to see behavior on individual sites. Any time a website, or third-party content embedded in a website, deposits a cookie in your browser, that cookie is confined to the cookie jar assigned to only that website. No other websites can reach into the cookie jars that don’t belong to them and find out what the other websites’ cookies know about you — giving you freedom from invasive ads and reducing the amount of information companies gather about you.