It sounds okay to me...
S2 /ˈfəːstli US -əːr-/ adv [sentence adverb]
used to say that the fact or reason that you are going to mention is the first one and will be followed by others
→finally, lastly lastly
Firstly, I would like to thank everyone who has contributed to this success.
Our first priority is to maintain the standard of work.
As I see it, my first responsibility is to my family.
firstly: you are mentioning about your first fact
first: you are metioning about the most important fact
So I believe, both sound okay
Student or Learner