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: firstname.lastname@example.org
Outgoing Mail Server (SMTP): smtp.mail.yahoo.com:email@example.com
TLS Certificate: [None]
Also your ISP doesn't keep up their DNS records. Even if you are not in the U.S .A, Canada or Mexico don't worry. A free OpenDNS.com (they're in several countries) account you can control your own DNS (see [yrl=http://www.opendns.com/support/videos/basic/]this video tutorial[/url]) and block known Trojan hosting web sites.