With "Slim" PS3 systems - and some newer "Phat" models - we use the E3 Flasher to downgrade the NOR memory.
It is a complicated process - and involves opening your system and installing hardware to tiny components on your main board. This is the only way for PS3 systems updated past 3.55 to be modified. Period.
make sure you have a nor chip ps3 that can be downgraded
for fat console,the highlighted is downgradable fat PS3 version
for slim console
You can check to see the lowest firmware by using the minimal downgrade check PUP file. Just rename to PSUPDAT.PUP and place in a folder named UPDATE, then place that in a folder named PS3, and put on the root of your flash drive. Put in in the PS3 and navigate to system update. Choose update from storage device. Don't worry it won't update your ps3. It will tell you what the lowest firmware your console can go