Gone through this pain today, and couldn't find any adware or spyware that would detect it. What I did is I managed to keep on clicking on Tools then on Addons, then looked for "play your piano" is and clicked Uninstall. Afterwards I clicked on Restart firefox.... all this while unwanted windows infinitely were loading, so I had to ctrl-alt-del ultimately, but the "annoying thing" finally left my browser and am happy about it.

Don't trust it or its maker again, as ad-filled things are not the answer for good add ons.

You may have set an open with action for a file extension or MIME type on that website that you need to remove.

See:

• Firefox repeatedly opens empty tabs or windows after you click on a link