Ubuntu: USN-3391-3: Firefox regression

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)