After the logo change, Yahoo! made another re-design. This time the mailbox have changed! I think is the third interface change on the last years. Lot of petitions are open to bring back the old design. If you are also not satisfied with the new Yahoo! design, there is a trick how to roll back.
Over the Settings menu, select Preference.
In Preferences, look for the “Viewing email -> Mail version” setting. Change it to Basic.
If you have change your mind there is also a button to: Switch to the newest Yahoo! Mail.