Looking on the internet deeply has found these results:

at first is the most popular phrase on the web.

firstly 10 results on the web	at first 1331,000,000 results on the web More popular!
Some examples and use cases from the internet:	Some examples and use cases from the internet:
Europe must firstly set an example and contribute to discussions. branding. Your fiancée, firstly escaped from the Pescadero Mental Hospital. This includes, firstly, genuine authorisation for multi Innovative activity involves, firstly, research and development. Gradac is firstly mentioned in 1649. He married firstly Isabel de Vignacourt and secondly Marie de Sennarpont. As suggested by the name, natural dyeing firstly involves making dyes from naturally occurring ingredients.	It doesn't look like this phrase is very popular! The search could not find any examples on the internet.

firstly

10 results on the web

at first

1331,000,000 results on the web

More popular!

Europe must firstly set an example and contribute to discussions.
branding.
Your fiancée, firstly escaped from the Pescadero Mental Hospital.
This includes, firstly, genuine authorisation for multi
Innovative activity involves, firstly, research and development.
Gradac is firstly mentioned in 1649.
He married firstly Isabel de Vignacourt and secondly Marie de Sennarpont.
As suggested by the name, natural dyeing firstly involves making dyes from naturally occurring ingredients.

It doesn't look like this phrase is very popular! The search could not find any examples on the internet.