Safari's third-party cookie blocking is not the same as Firefox's, last I checked.

Firefox just blocks them. Safari blocks them unless you visited that site in a first-party context. Shipping the thing Firefox implements breaks a lot more than what Safari ships.

So for example, if you visited facebook normally, third-party Facebook cookies won't get blocked in Safari.

Firefox _could_ ship the same thing as Safari. The upshot is entrenching existing monopolies, because only big enough players are then able to track you via third-party cookies.... This is the main reason Firefox hasn't done this, as far as I know.

Disclaimer: I work on Firefox, but not on the cookie bits.