If you can get to the service processor menus, you can update it from floppy disks. (I think the floppy drive is under the covers on the H80, next to the CD / Tape drive bays - undo the thumb screws in the corners of the front panel and lift it away) The firmware readme does give details of the floppy method.
If you can't even get to the service processor menu (connect an ascii terminal and hit a key while the operator panel shows OK - you may need to power it off to get to OK, remove all power cords for a few seconds) then you will need to drain the NVRAM. (The problem you now have is that AIX 5.3 has tried to rewrite the NVRAM but in a format that old firmware does not understand) To drain the NVRAM you need to power off the whole system (remove all power from the CEC and I/O drawer by removing the power cords) then open up the I/O drawer (the bit with the op panel, CD, tape, adapter cards, etc, and remove the battery for about 15 minutes. Then refit the battery and reconnect power, it should then come up to OK. Then you can use the floppy drive method, or install CD / Diag CD but I think these will both need the floppies still.