Score

Hm... I'm not going to add to the list of books, because it's a very specific selection of 15. However, I do have some comments and some suggestions.

First of all, some of these are nothing at all like Firefly.

Downbelow Station is one of my favorite books of all time, but Firefly was a space western, with humor; it could be by turns ludicrous, serious and sometimes ironically self aware.

Downbelow Station on the other hand is a vast, gritty, political novel about a terrifying war for humanity's future. A novel which gazes into both the large scale and the intimate horrors humans can visit upon one another.

(Some of Cherryh's other works come closer to the ideals of Firefly, such as: Merchanter's Luck, Finity's End, or Tripoint. All of those take place in the same universe as Downbelow Station, but Downbelow Station is the big picture, the canvas of the era. Whereas the other stories I've listed are more focused on the individual).

For people who liked Firefly, I would instead suggest looking to the heart of the story, rather than the (space opera) trappings. I would suggest looking at Chris Wooding's Tales of the Ketty Jay series. Those fall into the "Steampunk" sub-genre, but are much more faithful to the style and substance of what the Firefly series was. That is: A group of misfits, thrown together by fate, struggling to survive, a series laced with humor, grand ideals, pragmatism and ridiculous misjudgments, much like the Firefly series.