In one issue of the X-23 miniseries Target: X, Laura Kinney (X-23) and Megan are at a movie theater, and one of the kids seated behind them can be seen wearing a shirt with the Firefly logo.
Live Action TV
There are multiple references, shout outs and actor allusions on Castle.
Castle dressed up as Captain Reynolds for Halloween, referring to himself as Space Cowboy.
Similarly, there are multiple shout outs and actor allusions on Chuck, which costars Adam Baldwin.
Sheldon and Leonard of The Big Bang Theory were fans of the show and they supposed it would be on for years.
They put it in their room-mate agreement that they on Friday night, they would be watching Firefly.
Sheldon once said that Nathan Fillion is loveable and he thought him a great candidate to play a superhero.
The owner of Fox is on Sheldon's list of traitors and evil people... because Fox cancelled Firefly.
In the Community episode Advanced Documentary Filmmaking, Troy mentions that he and Abed have a pact that if one of them dies, the other will stage it to look like a suicide caused by the cancellation of Firefly.
The series gets a nod in Darths & Droids when one of the characters says "gorram." A bit of Fridge Logic when you consider that Darths and Droids is set in an Alternate Universe where the Star Wars films never happened, considering the heavy influence the films had on the genre which would have even affected Firefly.