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.
Apple can do this, because they have a limited yet consistent market share that mostly consists of their own customers.
But Firefox? If they block third-party cookies and Chrome does not, they might just end up losing even more users.
Websites can be changed to work without third party cookies. I'm blocking cookies since years and I have come across less than 10 sites that require it. When that happens, most of the time I just go elsewhere or I'll send them an email.
There are also lots of one-off cases where developers split functionality across multiple domains in ways that were fine at the time but now get blocked. These are all fixable, unlike the iframing case, but it's still a lot of sites.
Breaks because it was abusing lack of FPI.
Breaks innocently.
<snark>So, nothing you'd want to use.</snark>