Download Firefox over HTTPS, check for /etc/debian_version

The setup script was not running on my Debian Wheezy system as it
did not have an /etc/lsb-release file, or a matching file with a
DISTRIB_ID line.

The firefox binary archive was being downloaded over http without
any cryptographic verification.
Merge pull request #564 from DonnchaC/setup_script

Download Firefox over HTTPS, check for /etc/debian_version