How about 'there's no good replacement yet'? All that the firmware should be doing is making sure the hardware is intact enough to load the next stage, then fin

How about 'there's no good replacement yet'? All that the firmware should be doing is making sure the hardware is intact enough to load the next stage, then finding, loading, verifying (a the very least, checking signatures for accidental modification, and ideally checking the keys against ones that require physically moving a jumper or dip switch to change), and executing the first on-disk stage. MAYBE initializing a very basic text mode so it can show errors, MAYBE a halfassed keyboard handler to let you help it out if it gets confused. Granted, this does mean it has to have some sort of bus it can use to talk to various storage devices, but even this should be as simple as possible - you can switch to a more complex but faster one later, you just need to be able to load a couple kilobytes.