Bash – Ubuntu bash scripting with root commands

This will return true if a variable is unset or set to the empty string ("").

if [ -z "${VAR}" ];

First off, the script should start with the hashbang line:

#!/bin/bash
# All other lines should follow this one.

The LSB error relates to this block that's on all the system init scripts

### BEGIN INIT INFO
# Provides:          defaultdaemon
# Required-Start:    $remote_fs $syslog
# Required-Stop:     $remote_fs $syslog
# Default-Start:     2 3 4 5
# Default-Stop:      0 1 6
# Short-Description: Start daemon at boot time
# Description:       Enable service provided by daemon.
### END INIT INF

I haven't used update-rc.d before, but a quick look at the man page indicates that the default is S20 - this is the sequence in the boot that the script will run (where 1 earliest, and 99 is highest). As a general rule of thumb, this should be 99 unless you know otherwise, since what you're starting may depend on other services - mail, networking, syslog etc.

When you say the script doesn't work, what behaviour did you expect? This script is trying to start firefox while the machine is booting, it won't start it when users login.

You can set applications like firefox to run on login from the startup applications in the menu.