3rd party cookies make tracking users easier when the same cookie can be used on many websites.

Firefox does 2 things to protect you from that: it blocks known trackers cookies by default; and for the others it isolates them per domain so that kind of tracking doesn’t happen. That ensures you’re not tracked and at the same time it doesn’t break any functionality.

If you want to completely block them you can. There’s more info here: https://support.mozilla.org/en-US/kb/third-party-cookies-firefox-tracking-protection