If you are using Firefox browser, then just read out these points and implement these steps and also Get help through Yahoo Customer Care ServiceGet help through Yahoo Customer Care Service for removing or blocking ads on Yahoo account.
• To begin with, in your Firefox browser, just enter the small button with 3 horizontal lines of the Firefox window.
• After that enter Add-ons
• Now, In the Add-ons Manager tab, enter on Extensions.
• With this, in the search box, just type "yahoo mail ads" and then press Enter.
• And you will see the list of relevant extensions.
• You have to locate and install the one that blocks Yahoo Mail ads.
• Enter on the Install button for installing your chosen extension.