According to some people, each of the following three quotations, the first from a leading ninteenth-century intellectual, the second from a twentieth century scientist and the third from a noted 'author, playwright, journalist, composer, and public speaker' (Wikipedia), all three British, break not one but two 'rules', or at least conventions.
John Henry Newman (Cardinal Newman) - Certitudes in Religious Assent, published in The Dublin Review, Vol XVI, London and Dublin, 1871
The net results are, firstly that it would be best to drop the term race from our vocabulary, both scientific and popular, as applied to man ; and secondly, and more importantly for our present purpose, that until we ...
Julian Huxley - Eugenics and Society, Galton Lectures 1936
For two reasons. Firstly because it showed an abysmal ignorance of economics. ... Secondly, and much more importantly . . . . . . the 'What- does-it-matter-who-has-them?' school ignored the one thing that it is fatal to ignore in any virile nation.
Beverley Nichols - Men do not Weep 1941
In this post I take a look, from a largely British point of view, at the use of firstly, secondly (etc) and more importantly, to introduce clauses, and at the objections to them.