Go to Yahoo Mail. At the top left, next to Sign Out, there should be a button for Options. Click this and you get a drop down box with Mail Options.
In email options, put a check mark in the box next to InBox Show on Startup. This will open Yahoo Mail directly into your email Inbox folder.
You might also look to see if the box next to Social Features Enable Updates has a check mark in it. If so, remove it.