Capricious, The firmware is most likely for a player with a different screen type than yours so the wrong display driver is being used.
Have you adjusted the brightness setting as high as it will go?
The other thing you could try is FirmwareModify which is a small program that allows you to increase the brightness setting.
You can download it here
http://www.mympxplayer.org/firmware-modifier-df17.html
Load your firmware and move the brightness slider to the right, save and load the firmware to your player. This will not work on all players but worth a try.
Firmware Modify has a small bug that removes games and text reader from your firmware if you try to adjust the FM Radio setting so don,t try adjusting that.