firstboot: 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 CentOS possible default full paths are:

/usr/sbin/firstboot - from package: firstboot

You may install the required package with command yum install {package}

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