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This is an extension that blocks advertisements, and with the proper configuration will block all of the banner ads that appear on your Yahoo! Inbox.  Click the Firefox Menu button (☰). Select "Add-ons". Search for "Adblock Plus". Click the Install button next to Adblock Plus. Simply installing Adblock Plus is usually enough to get rid of the ads on Yahoo! Mail, but there are a few settings you'll want to double-check to make sure that it's properly blocking all the ads.  Click the "ABP" button next to the Firefox Menu button. Select "Filter Preferences". Ensure that "EasyList" is checked. Uncheck "Allow some non-intrusive advertising". If you've left your inbox open while installing Adblock Plus, you'll need to close and reopen the browser for Adblock Plus to take effect. "Yahoo Mail Hide Ad Panel" is another popular extension that just blocks the ads on your Yahoo! Mail page. The difference between this and using AdBlock is that "Yahoo Mail Hide Ad Panel" will reclaim some of the empty space left behind by the ad. You can install "Yahoo Mail Hide Ad Panel" using the same method as Step 1. This extension is only available for Firefox.

Summary:
Download the Adblock Plus extension. Configure Adblock Plus. Reopen Yahoo! Mail. Try a different extension.