This add-on hides the Ads panel in the right side of NEW (Oct. 2013) Yahoo! Mail’s Basic version, to give you a full-width Premium mail reading experience for free! The other option, of course is to pay $50 a year to Yahoo! Inc. Now, I am not one to usually promote hiding ads (because without them this website wouldn’t have a reason to exist) but for those who want it, here it is.
Here is more about the add-on from the developers:
Recently, Yahoo! Mail revamped their look and functionality to make it more streamlined (and made it look a lot like Gmail) but they maintained their wide advertisement banner (/panel) on the right side of the page, spoiling the whole email reading experience, especially for those who have big monitors. This simple extension automatically fixes that, so that the mails use the full width of your monitor rather than a fraction of it.
Want to learn more? You can grab Yahoo Mail Hide Ad Panel from the Firefox Add-ons website.