xpi-unpack: 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-unpack - from package: mozilla-devscripts

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

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