Downgrading your PS3 firmware from version 4.89 to 4.88 is a process primarily used by the homebrew community to regain access to specific custom tools or more stable "Custom Firmware" (CFW) builds.
Before we dive into the downgrade process, let's discuss some reasons why you might want to downgrade your PS3 firmware: how to downgrade ps3 firmware 4.89 to 4.88