mozilla-plugin-config: command not found

Have You tried accessing the binary with the full path?

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/mozilla-plugin-config - from package: nspluginwrapper

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