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

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

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