There were apparently several comic book series done in the Firefly universe. I don't know whether they're considered canon or what. I have not read them.

kagis

Apparently yes.

https://en.wikipedia.org/wiki/Serenity_(comics)

*Serenity* is a line of comic books published by Dark Horse Comics from 2005 to 2017. It is a canonical continuation of Joss Whedon's *Firefly* television series and the 2005 film *Serenity*, which are all part of the Firefly media franchise.[1] It was not an ongoing series; rather, it consisted of a number of miniseries and one-shots, released sporadically.

Starting in 2018, Boom! Studios began publishing its own line of Firefly comics.

https://en.wikipedia.org/wiki/Firefly_(Boom!_Studios_comics)

Firefly was an ongoing line of comic books published by Boom! Studios from 2018, set in the universe of the Firefly media Franchise. Written by Greg Pak and illustrated by Dan McDaid, this series submerges into the themes of family, loyalty, identity, and redemption focusing on the early experiences of Malcom Reynolds during the war that shaped his future as captain of the Serenity.[1] It is a canonical continuation of Joss Whedon's Firefly television series, the 2005 film Serenity, and Dark Horse Comics' Serenity comics, which are all part of the Firefly media franchise.[2]