just brought my ps3 320 gig CECH-2502B, it came with 3.50 fw preinstalled. i get to the factory mode just fine via AVRKEY doogle
but when i put in file 1 n mod 3.41 fw it results in constant blinkin of the green light on the ps3. even after an hr nothing happens so i'm forced to turn it off.
I suspect that CECH-25 with OFW3.50 preinstalled have minor changes in SYSCON eeprom, chipset configuration and/or BD-Firmware wich OFW3.41 is not aware of and unable to handle that - resulting in a panic or loop during boot. So you'll have to wait until this is found out and probably patched in a CFW.