A post (Comma after "of course" at the beginning of a sentence?) gives a clear rule/suggestion about the usage of comma after the transition words/phrases at the beginning of a sentence.
... one should use commas after their transition word or phrase that starts a new sentence.
However, the example sentences in Cambridge Dictionary seem to invalidate this rule.
Firstly I would like to thank you for your kind offer of a job …
Firstly the sodium chloride is dissolved in the water and heated gently.
Is the comma in this case required, optional or unnecessary? Could someone please give a hint? Thanks in advance.