Latest update fixes the major problem, which is that this used to not actually work in Windows :) It does now. The behaviour you see on Firefox for MacOS is replicated under Windows, giving the same experience you get with Edge (if you hate yourself enough to use Microsoft's browser).

Four stars instead of five because it seems incapable of handling one-time codes for MFA, so you'll still need to dual-wield either your phone or the iCloud for Windows Passwords app - but for most things it's now a seamless experience.