Maybe this will help someone.
I attempted to apply ljM506_fs3_fw_2308955_579078.bdl to several M506 printers via WebJetAdmin but after the update, the printers would only display a red circle with a bar and "Error". To recover from this problem, just re-apply the update via a USB stick (formatted FAT32).
Copy the firmware to the root of the USB drive (FAT32), insert the drive in the printer, power on the printer, press the "X" on the keypad as the printer boots to get to the Pre-Boot menu, select "3 Administrator", then select "1 Download", then select "3 USB..." and choose the BDL file. It will copy the file and when finished, the printer will display "Complete". Power off the printer and remove the USB stick. Then power on the printer and it will apply the firmware package (it took ~10 minutes).
While troubleshooting this issue, I formatted the "eMMC" in the printer (it didn't have hard drive). This led to an error 99.39.67 on the printer at startup. I followed the instructions above, but after letting it "Initialize" for 18 hours, I decided to try something else. Found a reply from Edison24h that suggested unselecting "Embedded" in the StartOptions (Preboot menu, Administrator, StartOptions, and uncheck "Embedded" (see https://h30434.www3.hp.com/t5/LaserJet-Printing/HP-M604-stuck-on-initializing-after-firmware-update/m-p/6717613/highlight/true#M346655 ).
If this helps you, please make a donation to your local historical organization. If it doesn't help, plan to visit a museum to take you mind off printers!