fb_config: 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 Debian possible default full paths are:

/usr/bin/fb_config - from package: firebird-dev

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

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