Unfortunately, it breaks core functionalities of yahoo mail.
I think this extension is too aggressive!
Indeed, it does hide the panel at the right, but however,
this panel is used also to show contacts, calendar and
previews.
After installing this extension, the right panel is always
blank when you open it, regardless of the content it should
display, not only ads!
This means you can't see your contacts or your calendar and
also, you can't preview an image or document from email
attachments.