Say you're making an argument that has two parts:
Why did such and so happen? First, bla bla. Second, bla-pity bla bla bla.
I'm thinking "secondly" would sound better. But if I use "secondly," do I have to use "firstly" -- which sounds so -- yuck.
The tone of the piece of writing is medium formal. I mean, it's formal, but not formal at the level of a brief submitted to the Supreme Court.