I have this problem as well!

This can happen if you have a Firefox extension or other (ad-blocking) software that has modified a file request and removed appended GET data (?xxx=xxx) from a link to retrieve a file.

Try to clear the cache:

• Firefox > Preferences > Advanced > Network > Cached Web Content: "Clear Now"

"Remove Cookies" from sites causing problems:

• Firefox > Preferences > Privacy > Cookies: "Show Cookies"

You will have to change your email password if mails are send out from that account and possibly also do a malware check on your computer.

• http://www.google.com/search?q=mac+malware+scan
• https://itunes.apple.com/us/app/bitdefender-virus-scanner/id500154009?mt=12

Thanks, Cor-el!

   I had to, finally, clear all cookies to get rid of this notice.  Luckily, I never opened it, so I don't expect it has hijacked mt email address book.

You're welcome.