firefox: 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/bin/firefox - from package: firefox

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

On Debian possible default full paths are:

/usr/bin/firefox - from package: iceweasel

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

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