My interpretation is that he probably used the same term as Suarez did and the ref heard it.
I think the ref was going to book Holgate and then decided he couldn't book Firmino for racism, and ended up pleading ignorance to it all. He went to 4th official to document that a racist comment had been alleged, but if he hears it, the answer is to send Firmino off. When Holgate makes the accusation, if he's heard something different, explain it to him, he was in the middle of them.
There will be other video footage than what is available to the public, and the ref's mic doesn't have to be activated to be recording. So I think it depends on how the powers want to handle it. But once they start asking Firmino for an explanation I think he's already done for by precedent.