I use the AdBlock Plus extension to do this - when you log in to your Yahoo Mail, wait for the annoying overlay to appear, then click on your AdBlock Plus icon. Select the option for "block element", then (left) click on the overlay. AdBlock will then block that element of the pop up. There are several of these elements in this pop up, so you need to repeat the process of selecting "block element", clicking on the parts of the overlay which are still visible. When I did this, there was one final "hidden" element which you can't see but stops the screen from being responsive. Again, just select "block element", click anywhere on the unresponsive screen and that should be it. Once you have your normal, fully functioning yahoo mail home screen back you're fixed. AdBlock Plus will remember to block the overlay in the future and you won't see it again.
I believe there's also a Chrome extension called "overlay blocker" which can do the same thing, although I haven't used this myself.