Do you use Yahoo Mail?  Do you like classic but Yahoo made you switch?  Here’s a work around that will get you back your classic interface.  I’ve seen posts regarding disabling javascript as well as changing resolutions, but that just seems like too much work for me.  Instead, there is a much simpler method.

Get Classic Back

1. Sign into Yahoo Mail normally.  You’ll get the “new” interface, which is what you don’t want. 
2. After you sign in, now you can go here: http://us.mc1811.mail.yahoo.com/mc/welcome?ymv=0 to get the Classic interface.  As you’ve already signed in, it passes the necessary credentials of the session over to the page that deals with classic.
3. (Optional) You can easily create a shortcut on your desktop to the Classic link and voila! You now have Classic back.  Please note though that messaging services within mail have been disabled in Classic earlier this month.