It is unlikely that your printer will support downgrading its firmware version as firmware is generally not designed to do that. The only way to normally downgrade firmware is to replace the formatter and upgrade to the previous version. New formatters often cost as much as a new printer so its usually not a valid troubleshooting solution to try.
Since you mentioned that this problem is firmware related you might want to give HP a call. Even when printers are out of warranty HP is willing to investigate firmware related problems from time to time. If HP can determine and confirm that your problem is firmware related they should be able to help you.