Mozilla developer Jeff Walden reported that in Gecko's implementation of
ECMAScript 5 API's enforces non-configurable properties with logic specific to
each API.  Scripts that do not go through these APIs can bypass these
protections and make changes to the immutable properties in violation of
security protections. This could potentially allow for web content to run in a
privileged context leading to arbitrary code execution.

External Reference:

https://www.mozilla.org/en-US/security/advisories/mfsa2015-109/