Switch Firefox to the latest release candidate

build5 contains a mozjemalloc fix but we're using bundled
jemalloc 3.6.0-204-gb4acf73. So, no PORTREVISION bump
for native version. Language packs only contain changes for
timestamps or slave hostnames aka build noise.

Changes: https://hg.mozilla.org/releases/mozilla-release/rev/9879336c72be
MFH: 2015Q3
X-MFH-With: r393690