xpi-pack: 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/xpi-pack - from package: mozilla-devscripts

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

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