I think that he is suggesting to using Yahoo Messenger from the browser. On the
Yahoo homepage it will say Messenger somewhere (depends on whether you have
customized the homepage where it actually is). Mine is with the tabs for mail
and calendar, etc. If you are logged in your Yahoo account on your home page you
will be logging into the Yahoo IM. When you click on it it will ask you to
install the Yahoo IM client, but if you ignore this and click at Get Started at
the bottom it will take you to the login screen.

Another way to go would be to use the Yahoo toolbar in Firefox or a service such
as Meebo which gives you IMing for many clients (MSN, Gtalk, Yahoo, ICQ, etc.).
Meebo works the same as Pidgin, but from the browser window. There is a Meebo
Add-on for Firefox 2, but I don't think that there is one for Firefox 3. It
works without a Meebo account, but if you signup for a free account your Meebo
password will log you into all of you clients with just the one password.

Also there is more than one Linux client. You could install Kopete which is a
KDE app, but it works in either Gnome too. The features of Kopete and Pidgin are
pretty much the same and it is a question of personal preference. I use Pidgin
because it is what I prefer, but Kopete works just fine. The KDE4 client for
Kopete is nice, but useless unless you want to install lots of KDE4 libraries.

Linux is all about choice and there is lots of it. If one thing does not work,
try something different.