first‧ly S2 [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]:
Firstly, I would like to thank everyone who has contributed to this success.
see usage note first2WORD CHOICE: WORD CHOICE:

first, first of all, firstly, at first
Use first, first of all, or firstly to introduce the first item in a list of two or more points, instructions etc First, make sure the screws are securely fixed in position. Then attach the wire. The plan was not practical, firstly because of the cost, and secondly because local people did not support it.Use first or first of all to say what happens first in a series of actions First I fed the baby. Then I made myself a sandwich.Use at first to say what happened at the beginning of a period of time, when this changed later At first I was nervous, but I soon started to relax.See also first

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