Earlier quoted context omitted.
I would think it would be Firefox which would have to support iCloud for Windows. If password managers could interfere with password fields in Firefox without its help, malware could do that, too. Or is there a generic password manager API on Windows that Apple doesn’t implement?
1Password, for example, has a plugin for Firefox. Something that you would expect in MVP for any password manager.
1: https://en.wikipedia.org/wiki/Usage_share_of_web_browsers