If your goal was to get back to a "clean" 4.88 Official Firmware (OFW): Install the 4.88 CFW first using the steps above. Once on CFW, the system restrictions are gone.
These cannot be downgraded to 4.88. You must use PS3HEN instead of CFW. Step 1: Check Compatibility with MinVerCheck how to downgrade ps3 firmware 4.89 to 4.88
Before attempting a downgrade, you need to know the "Base Firmware" your console shipped with. If your base firmware is 4.89 or 4.90, you cannot go back to 4.88. Download the tool. If your goal was to get back to a "clean" 4