Yep folks. Dark Alex skipped 4.02 and released 5.00 M33. Interested in the changes? Read the PSP 5.00 Firmware changes.
Requirements:
- PSP Phat or PSP Slim with 4.01 M33.
- PSP battery that’s at least 78% full
- Download: 5.00 M33 | 5.00M33 (RapidShare)
- Download: 500.PBP | 500.PBP (RapidShare) | 500.PBP (Official Sony Mirror)
- Disable ALL plugins.
Manual Installation of 5.00 M33:
- Extract the 5.00 M33 installer files to your computer.
- Copy the Update folder to X:/PSP/GAME/ on your PSP’s memory stick.
- Download the official 500.PBP. If the file you’ve extracted is named EBOOT.PBP change it to 500.PBP before proceeding.
- Copy the 500.PBP to the directory; X:/PSP/GAME/UPDATE on your PSP’s memory stick.
- Run 5.00 UPDATE from under Game on your PSP’s XMB
- Press X if asked to correct idstorage keys
- Continue to install the official 5.00 SONY when prompted
- After all process is complete, press X. Your PSP will reboot.
- Congratulations, you’ve just installed 5.00 M33
Plugins incompatible so far:
CXMB
CWCHEAT
POPSLOADER
Important Note: Dark_AleX has released a statement regarding the 0xFFFFFFFF PSOne error in 5.00 M33. See more details from the source 
Source and Pictures from atmaxplorer