fbserver: 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/fbserver - from package: firebird-superserver

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

On Debian possible default full paths are:

/usr/sbin/fbserver - from package: firebird2.5-super

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

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