If the ps3 is asking For that firmware and have tried to downgrade, then I take you put it in service mode buy using a dongle or something to get it into service mode by putting it I the USB closest to the Blu-ray and press power button followed by the eject button right away.
It will then shut it self off and then put the step 1 lv1 file and the Cfw you would like to use on an USB and put it in the USB closest the Blu-ray. Wait for it to turn off and then put the lv1 file for step 2 in and then it will boot. Is this what you are doing. Thanks