Remembering Jane Straus | May 18, 1954–February 25, 2011 | Author of the original Blue Book of Grammar and Punctuation
As noted earlier, few people say "firstly," and fewer yet say "fifthly," "sixthly," "seventeenthly," etc. Many adverbs do not end in -ly. It makes more sense to use second, third, and fourth rather than secondly, thirdly, and fourthly.
Letter A - B - C - D - E - F - G - H - I - J - K - L - M - N - O - P - Q - R - S - T - U - V - W - X - Y - Z