Really easy and quick page her to show you the default location to find the firefox directory in an ubuntu system:

MOZ_LIBDIR=/usr/lib/firefox

I hope this helps:

#!/bin/sh

set -e

# Firefox launcher containing a Profile migration helper for
# temporary profiles used during alpha and beta phases.

# Authors:
# Alexander Sack
# Fabien Tassin
# Steve Langasek
# Chris Coulson
# License: GPLv2 or later

MOZ_LIBDIR=/usr/lib/firefox
MOZ_APP_LAUNCHER=`which $0`
MOZ_APP_NAME=firefox