With Firefox open, I get the following box on my screen:
You have chosen to open: which is Plain Text
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:
"Remove Cookies" from sites causing problems:
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.
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.