firewall-config: command not found

Maybe Your $PATH variable doesn't contain the default paths? echo $PATH should give something like:

[root@server ~]# echo $PATH
/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin

If it's empty, You may set the PATH variable with this command

export PATH="/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin"

On Debian possible default full paths are:

/usr/bin/firewall-config - from package: firewall-applet

You may install the required package with command apt-get install {package}

Error in other languages:
firewall-config: bevel nie gevind nie
firewall-config: comandă negăsită
firewall-config: comando não encontrado
firewall-config: commande introuvable
firewall-config: command not found
firewall-config: käsku ei ole
firewall-config: không tìm thấy lệnh
firewall-config: komanda nerasta
firewall-config: Kommando nicht gefunden.
firewall-config: kommandot finns inte
firewall-config: komut yok
firewall-config: nie znaleziono polecenia
firewall-config: níor aimsíodh an t-ordú
firewall-config: no se encontró la orden
firewall-config: no s'ha trobat l'ordre
firewall-config: opdracht niet gevonden
firewall-config: parancs nem található
firewall-config: perintah tidak ditemukan
firewall-config: príkaz nenájdený
firewall-config: příkaz nenalezen
firewall-config: команда не найдена
firewall-config: командата не е открита
firewall-config: 命令找不到
firewall-config: コマンドが見つかりません