Ubuntu Security Notice 853-2 – USN-853-1 fixed vulnerabilities in Firefox and Xulrunner. The upstream changes introduced regressions that could lead to crashes when processing certain malformed GIF images, fonts and web pages. This update fixes the problem.
You can download this advisory from the following link: https://packetstormsecurity.com/files/download/82677/USN-853-2.txt
Source: https://packetstormsecurity.com/files/82677/Ubuntu-Security-Notice-853-2.html