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 notefirst2WORD CHOICE: WORD CHOICE: first, first of all, firstly, at firstUse first, first of all, or firstlyto 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 allto 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 alsofirst
Definition from the Longman Dictionary of Contemporary English Advanced Learner's Dictionary.