thunderbird: 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/thunderbird - from package: thunderbird

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

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