Using Adverbs in a Numbered List
Within the normal flow of text, it's nearly always a bad idea to number items beyond three or four, at the most. Anything beyond that, you're better off with a vertical list that uses numbers (1, 2, 3, etc.). Also, in such a list, don't use adverbs (with an -ly ending); use instead the uninflected ordinal number (first, second, third, fourth, fifth, etc.). First (not firstly), it's unclear what the adverb is modifying. Second (not secondly), it's unnecessary. Third (not thirdly), after you get beyond "secondly," it starts to sound silly. Adverbs that number in this manner are treated as disjuncts (