PS3 Homebrew Wiki
Master (v21.1b5)

Firmware Flashers

This method is very dangerous, and can result in a brick as you are directly modifying your system's flash memory. If you're okay with the risks, have done this before, or don't really care if you brick your PlayStation 3, go ahead and follow these steps. If you want to be on the safe side, I'd recommend using the ps3xploit or bguerville pages if you wish to prepare for CFW.
Firmware flashing involves directly modifying your console's flash memory (the NAND or the NOR chip) to downgrade. Before attempting, however, find out if your console has a NAND or NOR chip.