Switch to Basic Yahoo Mail

1. Mouse over the Settings menu icon | select Settings
2.  Click Viewing email.
3.  By "Mail version" select Basic.
4.  Click Save.
5.  Switch to Full-featured Yahoo Mail
6.  Click Switch to the newest Yahoo Mail in the upper-right corner.
7.  Clear your browser's cache.
8.  Restart the browser and return to Yahoo Mail.

to see video 

http://clkme.in/qUV1Cm