You really felt the need to judge him and his reasons to update the firmware? How sad ...
I have the same issue and after researching this I have realized a LOT of HP customers have the same issue with no resolution in sight. The printer will not accept the new firmware. The update procedure will terminate with a "Critical error 7" and then revert to "Ready 2 Download".
Seems a new "formatter PCB" or a new printer is the only fix.
To other customers: In case you want to seek out a new formatter unit (the "logic board" in the printer) the part number you are looking for in the LaserJet Pro 300 color MFP M375nw is this: CE855-60001.
If you don't have a large stock of toner/supplies for this printer, then perhaps a new printer might be a better idea since support for this model seems to be ... lacking.