first, firstly

First is the only form for the adjective.

• The first European settlers in North America were Vikings.

First is also the preferred form for the adverb in most uses.

• First, do no harm.

However, the adverb firstly can be used to begin lists and to enumerate statements, questions and topics.

• Foodborne illness is caused by three main factors:
  • firstly, cooking or storing food at improper temperatures;
  • secondly, allowing cross-contamination between raw and cooked foods; and
  • thirdly, failing to practise good personal hygiene.