USN-3391-1 introduced a regression in Firefox.
USN-3391-1 fixed vulnerabilities in Firefox. The update introduced a performance regression with WebExtensions. This update fixes the problem.
17 August 2017
A security issue affects these releases of Ubuntu and its derivatives:
USN-3391-1 introduced a regression in Firefox.
USN-3391-1 fixed vulnerabilities in Firefox. The update introduced a performance regression with WebExtensions. This update fixes the problem.
We apologize for the inconvenience.
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 conduct cross-site scripting (XSS) attacks, bypass sandbox restrictions, obtain sensitive information, spoof the origin of modal alerts, bypass same origin restrictions, read uninitialized memory, cause a denial of service via program crash or hang, or execute arbitrary code. (CVE-2017-7753, CVE-2017-7779, CVE-2017-7780, CVE-2017-7781, CVE-2017-7783, CVE-2017-7784, CVE-2017-7785, CVE-2017-7786, CVE-2017-7787, CVE-2017-7788, CVE-2017-7789, CVE-2017-7791, CVE-2017-7792, CVE-2017-7794, CVE-2017-7797, CVE-2017-7798, CVE-2017-7799, CVE-2017-7800, CVE-2017-7801, CVE-2017-7802, CVE-2017-7803, CVE-2017-7806, CVE-2017-7807, CVE-2017-7808, CVE-2017-7809)
The problem can be corrected by updating your system to the following package versions:
To update your system, please follow these instructions: https://wiki.ubuntu.com/Security/Upgrades.
After a standard system update you need to restart Firefox to make all the necessary changes.