The "person of reasonable firmness" is a hypothetical person, spathi. The phrase is used in many statutes. It is used to establish whether the behaviour of those involved could have caused a person of reasonable firmness to (for example) fear for their safety. There does not have to be such a person present or even likely to be present and there does not actually have to be such fear caused. It is a matter for the jury (or bench of Magistrates) to decide if this element of the offence was proved. In my view (again, with very limited knowledge of the facts) that this is where the charge of Affray failed.
It is often somewhat pointless picking over these cases where the verdict seems at odds with (what we believe) are the facts. Mr Stokes's trial took over a week, I believe. You can read all the pertinent information published about it in probably half an hour. The jury heard all the evidence from both sides and were directed by the judge as to what they need to find to convict the defendant.