I lost count of the Firefly references, but these are the ones I remember:
Castle saying "Shiny" in response to finding out about the murder at the Con
Mentioning "That Joss Whedon Show"
Jokes about Nebula-9 (the fake show at the Con) being canceled a decade ago after only twelve episodes
The villains on Nebula-9 were called Creavers (Reavers on Firefly) who would eat your face after you were dead, if you were lucky. This was almost a direct quote from the show.
One of my favorite scenes, though, had to be Armin Shimerman (Quark from Deep Space Nine) as the electronics whiz who built the phaser gun. I loved his "Han Shot First" T-Shirt.
Another possible reference to Firefly include the character who was murdered had bought the rights to Nebula-9 and had been making webisodes, trying to continue the series. There is a rumor that Nathan Fillion wants to buy the rights to Firefly to do just that.
Who else loved this episode?
*Yes, I know it aired a month ago. By choice, I don't have cable and we watch TV on Hulu or streaming at the network's site. Yay for technology! And the show was still cool.