I was browsing Facebook in HTTPS mode today and my Firefox browser hanged up. After a few seconds my download manager has 3 queued MP3 files; I thought its a virus.
I scanned my system using Windows Defender and Avast Antivirus but no signs of infection.
Then the same incident happened; this time I checked the properties of the mp3 file.
It was hosted on Facebook.
I continued to download the mp3 file and checked it via a hex editor. It turned out to be a GIF photo file.
A source told me that this was a bug in the Facebook photo system, and images (animated ones) can execute the animations if uploaded with .mp3 extension.