firewall-applet: 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-applet - from package: firewall-applet

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

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