You can go back to the old version by turning off Javascript temporarily. Some say you can't go back, but it's not true.
http://my-reference-notes.blogspot.com/2011/08/how-to-switch-back-to-yahoo-mail.htmlHow to switch back to Yahoo Mail Classic:1. Disable your browser's Javascript
Windows Internet Explorer
To disable JavaScript:
i. From the Tools menu, or the Tools drop-down in the upper right, choose Internet Options... .
ii. Click the Security tab.
iii. Click Custom Level... .
iv. Scroll to the "Scripting" section of the list. Uncheck to Disable scripting.
v. Close and restart your browser.
Mozilla Firefox
i. On the Tools menu, click Options.
ii. On the Content tab, uncheck check box to disable JavaScript.
iii. Click the Go back one page button to return to the previous page, and then click the Reload current page button to run scripts.
2. Open Yahoo mail URL and login using your id and password.
3. A warning message will be prompted that JavaScript is needed to be enabled for using the new Yahoo! Mail. You will also see two options to return to a previous version of Yahoo!Mail. Choose the first option to switch just for one time or second option to return to a previous version. Choose the second option here.
4. Now you are back to the classic version.
5. In order to access all web sites properly, don’t forget to Enable JavaScript from your browser (Internet Explorer or Firefox).