These cannot be downgraded to 4.88. You must use PS3HEN instead of CFW. Step 1: Check Compatibility with MinVerCheck
Since the official BGToolz site often fluctuates, find a reliable 4.89-compatible mirror. how to downgrade ps3 firmware 4.89 to 4.88
Always verify the MD5 hash of your firmware files to ensure they aren't corrupted before installing. These cannot be downgraded to 4
Some work, some don’t. You must check the "Date Code." Always verify the MD5 hash of your firmware
You can then take an official 4.88 update file and install it over the CFW.
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.
Since you are on 4.89, you cannot simply install a 4.88 update file over it; the PS3 will block it as an "older version." You must exploit the system memory (Flash) to allow the installation of older or custom software.