dh_xul-ext: 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/dh_xul-ext - from package: mozilla-devscripts

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

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