I can assure you that your problem is not universal. I have no trouble accessing my free Yahoo! account from my MacOS X 10.4.11 computer, my OS X 10.8 laptop, my iPhone 4S, or my iPad.
Therefore, you may rest assured that your problem is local to your system. It is on your system where corrections must be made. If you need help making those corrections, then you must give some information about your system.
Assuming that you live in the US, then these are your proper settings:
Incoming Mail Server: imap.mail.yahoo.com
User Name:
myaccount@yahoo.com
Password:
Outgoing Mail Server (SMTP): smtp.mail.yahoo.com:myaccount@yahoo.com
TLS Certificate: [None]
Under the [advanced] tab
IMAP Path Prefix: (left blank)
Port: 993 Use SSL [checked]
Authentication: [Password]
If these are the settings that you are using, then you need to look at your security settings.