... Forgotten Worlds. ...
I don't see a reason to ever not be firing in that game.
To move your pod in front / in back. For blocking and getting around/through walls/floors/obstacles. That blocking
really helps.
The game internally already supports auto-fire (that is holding down the button generates a continuous stream of bullets) so an auto-repeating circuit wouldn't be needed.
Right, so you don't want "auto-fire". You want "continuous button press". "Auto-fire" is something different, and there are strong feelings on it; readers might get confused and give you views on the unrelated auto-fire instead of what you want.
Is there a way to get Mame to do the auto firing [continuous press]?
AFAIK, not without editing the code. For FW and what you want, I'd change the IPT_BUTTON1 to IP_ACTIVE_HIGH (from IP_ACTIVE_LOW, line 674 in mame 0.110u2) and recompile. This would make it so you would fire
unless you press the button, without effecting other games. It's a hack, though, and will not be accepted in official mame. Just like auto-fire.