Security researcher Holger Fuhrmannek reported that when the Mozilla updater
is run, the updater can be manipulated to load the updated files from a working
directory under user control in concert with junctions. When the updates are
run by the Mozilla Maintenance Service on Windows, these malicious files can
be run with elevated privileges and be used to replace arbitrary files on the
system. This could allow for arbitrary code execution by a malicious user with
local system access but does not allow for exploitation by web content.

This issue is specific to Windows and does not affect Linux or OS X systems.

Upstream bug:

https://bugzilla.mozilla.org/show_bug.cgi?id=1177861

External References:

https://www.mozilla.org/en-US/security/advisories/mfsa2015-100/