Hi there mate!
Sadly, there is no way to install a CFW on your system, as the last OFW that could be 'tricked' is the 3.55. Also, there is no way to dowgrade it's OFW to a lower one.
There is a chance to downgrade you OFW but you'd have open your unit and solder a Hardware Flasher to erase the FW and put a lower one. One thing I am not sure of, is that this Hardware Flasher works on systems that comes with a OFW greater than 3.55 from factory.
There is work underway among the PS3 scene that would find the keys to unlock any OFW, no matter it's version, and open up any console to any CFW, but there is no news about it at the moment... and may take a while. So stay tuned to the site!
Cheers and good luck!!!