I saw another report about this recently. This occurs when you are reading Yahoo email? I'm not sure there's a solution here, but please take a look: https://support.mozilla.org/questions/1378488 And more generally, this error usually occurs when a document is "framed" or embedded in a page. Try this workaround and see whether it helps: * right-click the area where the error message appears (or Shift+right-click) * look for "This Frame" on the right-click context menu * if you find that, expand it and click "Open Frame in New Tab" to pop the document out of the frame.

See https://support.mozilla.org/en-US/questions/1379845