If you're still having trouble with the methods already posted, here's an alternate method to get back to 5.50 OFW from GEN D3 5.50.
Requirements:
- Gen D3 installer (it includes options to revert to official firmware)
- 5.50 OFW eboot
1. Copy the Gen D3 installer to the GAME folder.
2. Copy the OFFICIAL 5.50 eboot to the root of the memory stick. Rename to "550.PBP"
3. Launch the Gen D3 installer on your PSP.
4. Select the option to "Flash Install OFW 5.50"
Note: it's NOT the option to "install any official Sony firmware."