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>