Cannot login to Apple Account with Safari but can with Firefox
Every time I try to login to my Apple Account using Safari, it fails. Whether I use the stored password, copy and paste from my password manager, or type the password in myself, the login just does that little sideways dance and refuses to accept it.
Using the exact same procedures from Firefox works every time.
Safari is Version 18.0 (19619.1.26.111.10, 19619). My M1 MacBook Pro is running Sonoma 14.7.
I have re-started Safari. I have re-started the Mac. Neither helped.
I found some related threads from 2020 including one about installing an extra certificate, which I did (and re-started the Mac again). No luck. At that time the consensus seemed to be that it was a back office problem at Apple's end, and nothing the client could do would fix it.
I found a thread about signing in to iCloud after changing the password, and followed it. No luck. I haven't found a current thread with my exact problem (but maybe I missed it).
Any advice?
This is entered from Firefox, because, obviously, I can't sign in with Safari.
MacBook Pro 16″