Do not open or run that file it is malware.

It may even infect your computer in an advanced way using malware loading into the registry and memory.

We ae trying to find out more about this. I am hoping within the next day or so Mozilla staff will offer advice about this. Please post back to bump this thread in two or three days to see if there is any more advice.

Meanwhile it would not harm if you scan your computer with the tools mentioned in

• Troubleshoot Firefox issues caused by malware

At the moment although we think it may be spread by some sort of malvertisment we are not sure why it seems to affect some users and not others.

I will tag this post with bug1282106 , that may help us keep track and pass on any new information.

To protect yourself against malicious .js downloads intended to damage Windows, you can set Windows to open .js files in a text editor instead of executing them.

The exact method for this depends on your version of Windows, but all recent versions should have a control panel where you can specify the program you want to open different kinds of files by their file extension. I have attached a screen shot from Windows 7, for example.

Edit: The following article has a much more thorough explanation. If you don't have a .js file available to right-click, however, you may need to create one. http://www.computerworld.com/article/3090146/windows-pcs/blocking-javascript-can-stop-some-windows-malware.html