I disagree entirely. It's not a magic word mate. It doesn't gain volume just because you're not paying attention. If you don't hear something you don't hear it. It's really that simple.
You're right, it could be a misunderstanding and I hope it is, and wasn't Suarez a really nice bloke too?
it's not a case of the referee not having heard what firmino was saying though. clearly he was close enough to hear firmino speaking. of course specific words do snap you to attention, i.e. if someone says your name or, in my case, if the missus says 'money' ! i think you would be alone if you are trying to say you havent been paying attention to something and then a word has caught your attention for whatever reason. n word is definitely an attention catcher in my mind.
thats just my perspective of course, but i do think if it turns out holgate is the only one to have heard it (and firmino doesnt admit to it like suarez did, key difference there), then it is extremely unlikely to be taken any further, partly on that basis that they think it would be unlikely it could have been said without anyone in hearing range not hearing it.