The official Mozilla blog post has already released a brief description of how the hack works and why you should upgrade to Firefox version 39.0.3.
Windows users are the most vulnerable as majority of them use Mozilla Firefox browser but this program searches specifically for developer based files. So, you may not lose your movies, audio and other programs but rather will lose any FTP files, .purple among other configuration files.
The same issue has infected Linux machines as well where it searches for global configuration files and relevant user directories. Mac doesn’t have any reports so far but it is not completely immune to an attack.
Even before the attack could spread, Mozilla Firefox has released a new patch update with this version.
You can read more about the technical details related to this vulnerability at the official Mozilla blog.