Many references are made. One such being the tattoo 'Serenity' across Holder's chest.
Holder's girlfriend, Caroline Swift who is played by Jewel Staite, also played Kaylee in the famous one season hit series Firefly (2002) (Joss Whedon), in which she is a mechanic for the cargo ship named 'Serenity'. The series had gained enough fan support and donation to later become a movie, titled Serenity (2005). Caroline makes silly remarks about Holder's tattoo, but requests he keep it.