Among the critical advisories is a fix for two memory safety errors in libGLES in the ANGLE graphics library. One of the bugs was a missing bounds check that led to memory safety errors when manipulating shaders. This could have led to writing to unowned memory. The second similar issue also affected shaders when insufficient memory is allocated for a shader attribute array. Both issues could have led to an exploitable crash.
A separate use-after-free vulnerability, reported through HP's Zero Day Initiative, involved HTML media elements on a page during script manipulation of the URI table of those elements. This, too, would result in a potentially exploitable crash.
The release also addresses five “high” level advisories, or those that contain vulnerabilities that can be used to gather sensitive data from sites in other windows or inject data or code into those sites with no more than normal browsing actions.
One of these bugs, CVE-2015-4505, allowed for arbitrary code execution by a malicious use with local system access when the Mozilla updater is run. It can be manipulated to load updated files from a working directory under user control, and when the updates are run by the Mozilla Maintenance Service on Windows, the file can run with elevated privileges and be used to replace arbitrary file on the system.
The new browser version also comes with added functionality for Firefox Hello Beta, a self-described “global communications systems built directly into a browser.” The new feature will now allow users to send and receive instant messages when they're in a Firefox video call.