USN-4599-2: Firefox vulnerabilities

USN-4599-1 fixed vulnerabilities in Firefox. This update provides the corresponding updates for Ubuntu 16.04 LTS. Original advisory details: Multiple security issues were discovered in Firefox. If a user were tricked in to opening a specially crafted website, an attacker could potentially exploit these to cause a denial of service, spoof the prompt for opening an external application, obtain sensitive information, or execute arbitrary code.