You can downgrade to 3.60 OFW but wouldn't be able to dehash as you can only do it on 3.55
Cfw is basically signed with 3.55 keys so unless you're on 3.55 you cannot install cfw period. Not even on 3.60 as we don't have the keys.
Downgrading patches are signed with 3.55 keys
Playstation 3 (PS3) E3 flasher CFW Downgrade Service (Latest Official Firmware to 3.55/Latest Custom Firmware) NOR Flash Only (UK)