Ack, sorry: that link should have been https://bugzilla.mozilla.org/show_bug.cgi?id=106400 – I was hastily searching bugzilla on a phone.

The iCloud issue is discussed towards the bottom and references the Chrome ticket to remove support:

https://bugs.chromium.org/p/chromium/issues/detail?id=466638

Thanks.

It sounds like the real problem here is the fact that iCloud keychain doesn't have an access control list so apps can only access items within their same App Group, which means Firefox can't see Safari passwords and, if Firefox switches to iCloud keychain, no other browser can read passwords from Firefox. Though honestly this doesn't sound like a big deal to me (especially since the "Firefox can't see Safari passwords" is already the situation).

The Chrome bug does claim that Mac apps could not use the keychain access groups entitlement without being in the App Store but I can't find any hint of that being true today. I believe Apple relaxed the iCloud restrictions at some point so it's possible that this used to be true and isn't anymore.

It mostly annoys me that we had at least 17 years where this relatively modest amount of code would have made a big security & usability benefit. Even if Apple eventually deprecated it, that’s a longer useful lifespan than an awful lot of code has.