It was just after that he called him a son of a bitch/whore that he reacts, which is when Kenny's fat head gets in the way, so we don't actually know.
It seems strange that no one else heard it, including the referee who was actually closer to Firmino, and that no other player reacts in any way.
I read something that said it was smoothed over as a misunderstanding at half time, but there will be procedure to follow.