So it won't allow you to boot recovery mode even after a firmware flash? I'm puzzled. Try having the phone off then hold the Power + Volume Up and Home button until you get the blue 'Recovery Booting' text and wait for it to boot recovery. Should take around 10-20 seconds.
Then you will have to do a wipe data / factory reset most likely in order to fix this boot loop issue. Make sure you are running latest firmware and not previous ones as this can cause issues if using too old of firmware.