If you don’t like the All-New Yahoo Mail design, you have the option to switch back to the Classic view. It’s actually pretty easy, but the option to do so is hidden away (presumably because Yahoo really doesn’t want you to use the Classic view). After you’ve logged into your Yahoo Mail account, look for the Options link in the upper right-hand portion of the page. When you click on it, a drop-down menu will appear:
Left-click on the “Mail Classic…” option and a new dialog box will appear on the screen asking you to confirm your selection.
Left-click on the “Switch to Yahoo! Mail Classic” button and you’re all set. Now you’ve switched your account back to the Yahoo Mail Classic view.
UPDATE: I have a new blog post on how to switch back to Yahoo Mail Classic. If you can’t make the steps above work for you, then this new method should would quite well.