Update: Custom Firmware 5.00 Prometheus-3 (v2) has just been released by liquidzigong to fix the booting back to the XMB issue.
Being a simultaneous release Liquidzigong has a version of Prometheus-3 for you die-hard 5.00 M33 users. Like the 5.03 and 5.50 Prometheus-3 releases. Custom Firmware 5.00 Prometheus-3 lets you run the latest 6.30/6.31 games on your older fabled 5.00 M33 CFW.
Just run the installer from PSP/GAME/ and hit X, your 5.00 M33 installation is now set up to run 6.30/6.31 OFW games. Liquidzigong comments, “Think of it as an unofficial Custom Firmware 5.00 M33-7″
Custom Firmware 5.00 Prometheus-3 changelog:
Improves the compatibility of CF with the latest games that require the official firmware 6.30/6.31