Switch back to Yahoo Classic Mail !!

First Method:
A •Safe Work Around• - in Firefox or Google Chrome Browser.
(Did Not Work In IE8)
Click "Tools".....click "Options"......click "Content"
UN-check/Disable •enable JavaScript•
click OK.

Log back in to Yahoo! Mail through Your Browser -- Switch to Classic Mail....
THEN RE-Enable your Java Script.
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2nd Method:
Lower your Screen Display Resolution:
When The Screen Resolution is too Low for the "New Yahoo! Mail" (formerly Beta),
A message appears asking "Do you want to switch to Mail Classic?"
Click ►"Yes"......
Once Done, RE-Set your Screen Resolution.

♦IMPORTANT NOTE -- Do The Screen Resolution in Stages!
A Screen Resolution less than the Original Setting causes Unstable Effects
in the way web pages and formats appears.

Best of Luck To You..