Mozilla released the version 39.0.3 Firefox to patch a critical 0-day vulnerability that has been exploited in the wild. The vulnerability resides in the PDF viewer, for this reason it does not affect Firefox for Android and other Mozilla products that dont contain the viewer component. The exploit is very insidious, it was developed to avoid leaving traces on the targeted system. The attack has been targeting certain types of files hosted on Windows and Linux systems. Users urge to change passwords and keys contained in the files targeted by the attackers.”]
Source: https://securityaffairs.co/wordpress/39198/cyber-crime/0-day-firefox.html