I was just wondering if rapid fire is allowed in speedruns

no its not

What is rapid fire? is it like a turbo button that mashes for you?

Yes

I know this is an ancient forum, but is it at least allowed on emulators?

if it's not allowed on hardware and can't be done on it without a third party controller or tool, it's definitely not allowed on emulators either. Also site rules ban such things aswell