Update: Custom Firmware 5.50 Prometheus-3 (v2) has just been released by liquidzigong to fix the booting back to the XMB issue.
Chinese hacker liquidzigong has also released an updated Custom Firmware Prometheus-3 version for users running 5.50 GEN-D3 or 5.50 Prometheus. You need to have a PSP-1000 or PSP-2000 non TA088v3 and running either a Prometheus Custom Firmware release or Custom Firmware 5.50 GEN-D3. This update has added support for games that require original firmware 6.31 in order to run.
Custom Firmware 5.50 Prometheus-3 changelog:
Improves the compatibility of CF with the latest games that require the official firmware 6.30/6.31
Instructions for installation:
Make sure your on 5.50 GEN-D3 (Download below)
Install the Custom Firmware 5.50 Prometheus-3 to your MS PSP/GAME/
Run the installer from the XMB and then hit X to start the installer
