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Just lower the page "resolution" to the point where it won't support the Beta version and you will be asked if you want to switch to Classic.
(start>control panel>appearances and themes>display>settings>>lower it and click OK).

Once you've changed your mail page back to Classic----go back to the control panel and return the page resolution to normal, (so things will continue to work smoothly).
How do I go back to my old yahoo mail? - Yahoo! UK & Ireland Answers

Eu am setat 800x600 si a mers.