YPOPs is another email "gateway" program that can be used to access Yahoo's free e-mail using Thunderbird. I don't use Yahoo's email, so I can't answer whether such a program is still required but a quick browse through my bookmarked sites on email usage would tend to suggest that it is. I found a descriptive site for YPOPs here:
http://email.about.com/cs/winw2previews/gr/yahoopops.htm