Check with MinVerChk to find out the lowest FW you can downgrade to:
Here is the simple steps you need to follow:
1 - Copy the MinVerChk to your USB stick, the same way as if it was an firmware upgrade.
2 - Insert the USB stick into the PS3.
3 - Start a firmware update like normal from XMB (Don't worry, it will not update!)
4 - It will shortly fail and display the Firmware Base Value