xpi-repack: command not found

Have You tried accessing the binary with the full path?

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

[root@server ~]# echo $PATH

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-repack - from package: mozilla-devscripts

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