Paul…Paul, Paul, Paul… I really do n’t think you understand what constitutes an adverb – at least, not on this occasion. Your description: “In a basic sense, adverbs are words that answer the questions How?, When?, Where?, How much?, Why?, and to What extent? (Yeager, Best Language Book: p. 3)” Yes, an adverb will usually answer such questions, but the core understanding of adverbs is that they modify verbs or other modifiers (adjectives and other adverbs). I ‘m honing in on how what they do distinguishes them.
You take this to mean that ‘firstly’, ‘secondly’, and ‘lastly’ are poor form. You may not like the sound of it, but your argument that ‘first’, ‘second’, and ‘last’ are adverbs is a bit unsound (to your credit, the likes of ‘Grammar Girl’ agrees with you). In sequencing, I can nearly agree with your point; it ‘s superfluous and I ‘d be unlikely to use them in such a context. Her argument, however, diverges from yours in that she sees ‘First’ at the beginning of a sequence referring to ‘the first point is’ ,in which case, we are dealing with an adjective.
I know I ‘m sifting too finely…but so are you. It should be easy to recognize that ‘first’ and its counterparts function primarily as adjectives, but may be used as adverbs also without the need to add -ly. I would much rather see you criticizing errors concerning function – you are n’t displaying much imagination in this aspect.
Finally, or the final point is, both you and Grammar girl bemoan where ‘common parlance supercedes proper usage’ as though written grammar is primary and spoken is derivative. Again, language serves communication – communication may be constrained by linguistic norms, but grammar should serve us – not the other way around. Your idea that common acceptance crystallizes errors is fair, but should n’t we primarily concern ourselves with that which blocks communication?