Not OP but I have exclusively used Safari since the first version and recently switched to Firefox, and the password manager not being able to use iCloud Keychain is really a major bummer.

For such a long time Firefox didn't take being a native Mac OS citizen seriously, with scrolling not being natively implemented, form fields feeling "off" and in general just feeling like a Windows app in a Cocoa window.

Luckily those days are long gone now, and Firefox now feels much more like a native app, save for a couple of dialog boxes here and there. But having used iCloud Keychain for such a long time, and there being no option to import my 500+ passwords, it really has been a pain in the ass to switch browsers.