> The web will break.

I have been using the First-Party Isolation feature for a few months now. FPI (privacy.firstparty.isolate) is a flag that was originally created for the Tor Browser project. It's a strict version of State Partitioning without any way to unpartition (no permission dialogs, no heuristics).

So far I have only come across one broken website: Microsoft Teams login through Okta SSO.

The rest of my logins worked flawlessly with FPI turned on. Nothing close to the 50% that you mention.