Haven't a clue where it's come from but shouldn't it be 'Firstly'if it's going to be followed by 'secondly' etc.? I'd say that 'first of all' is more likely to be followed by 'then'.
I am willing to be corrected if there is sound evidence for 'First of all'in the context you've used.
🦞 The Lockdown Gang still chatting 🦞
