With Windows 10 starting from Win7 separate users accounts don't mix unless they are using your same Firefox Sync login at the same time. So you need to change your FF sync login/pswd to prevent this from happening. Also changing and making the Admin/Owner account for updates only and not use it for everyday purpose to prevent malware/virus infections. Making limited user accounts to prevent security holes. There's nothing wrong with Edge it works just fine. Those fears are overblown.