How to Remove Yahoo Mail Chrome Extension Splash Screen
Please note that this post is over a year old and may contain outdated information.
Yahoo recently started displaying a large advertisement for their Chrome extension when you try to access your Yahoo Mail inbox (mail.yahoo.com). This advertisement covers the entire screen, and is shown when you first access Yahoo Mail and are using Chrome. If your browser clears cookies each session, you'll see it every time you access Yahoo Mail with a new browser session. This is beyond obnoxious:

Add the following filter to your ad blocker's custom filters section:
This will remove both the ad and the large semi-transparent black background.
To block this annoying advertisement, you'll need an ad blocker extension or plug-in for your browser. If you don't already have one, you can get one from your browser's extension/plug-in page. I recommend µBlock Origin for Chrome and AdBlock Edge for Firefox. Any one that supports the AdBlock Plus filter syntax will work.

How to Remove It
Add the following filter to your ad blocker's custom filters section:
###uh-dmos-wrapper
This will remove both the ad and the large semi-transparent black background.
Don't Have an Ad Blocker?
To block this annoying advertisement, you'll need an ad blocker extension or plug-in for your browser. If you don't already have one, you can get one from your browser's extension/plug-in page. I recommend µBlock Origin for Chrome and AdBlock Edge for Firefox. Any one that supports the AdBlock Plus filter syntax will work.