Wowland, If your player is being recognised as an ADFU Device it is in recovery mode and will not accept a standard firmware dump. You will need to use a full version firmware, there are a few on site
HERE.
Once you have loaded a full firmware your player should then appear as a removable storage device and you should be able to load standard firmware dump.bin.
Finding the right firmware may require some searching. I suggest you download s1res
HERE. You can use s1res to look inside the firmware and see the images used for menus, start and shutdown screens etc. Try and find ones that use the same images as your original firmware.
So just to be clear
1. Load a full version firmware to ADFU Device then format your player.
2. Download and check some firmware dump files with s1res.
3. Try loading any firmware you find that have similar images as your original firmware had.
Let me know how you progress.