Welcome to cdfreaks
Did you backup your firmware first? If not maybe some else with a DW-D26A can help. Yes you can flash it back if you have the firmware bin file.
In saying that I'm surprised that the firmware would cause this. Your drive is a 1673S with Sony firmware. The hardware should be the same.
1/ Are you burning faster?
2/ Are you using the same media?
3/ Is it +R? Have you tried bitsetting?
4/ What media are you using?