A major vulnerability discovered by Mozilla lurking in an advertisement shown by a Russian news site could steal your files and upload them to a Ukrainian server without you ever knowing.
Interestingly, the files it searches for on the local system are mostly developer focused. On Windows, the attack specifically looks for FTP configuration files, subversion, .purple and other account information. On Linux, it looks for global configuration files and user directories.
Mac users aren’t specifically targeted by the attack that was discovered, but wouldn’t be immune if targeted.
The attack doesn’t appear to be widespread right now, having only been spotted on a Russian ad network, but it’s likely only a matter of time until it spreads as more people discover it.
All versions of Firefox are affected and Mozilla says that to protect against the exploit you should update to version 39.0.3 right now. Enterprise users can patch to 38.1.1.