PSP Fat (100X) 1st PSP generation · Pandora Battery access: Yes · Downgrading possible: Yes · lowest Firmware: 1.00 · Permanent CFW available: Yes

Result: You can install a full Custom Firmware of your choice!

Note: You might consider installing a 6.60 CFW since most plugins,
themes and programms are not optimized for 6.61 as of yet!