Wikipedia is wrong. That is the order of the DVD set, but the internal chronology of the later and un-aired episodes yield this order:
1–10. Serenity through War Stories (as on the DVD and Netflix)
11. Heart of Gold [#13 on the DVD & Netflix] (Inara plans to leave Serenity)
12. Objects in Space [#14 on the DVD & Netflix] (reveal of River’s psi abilities; Mal & Inara talk about Inara’s plans to leave)
13. Trash [#11 on the DVD & Netflix] (the crew make explicit reference to River’s psi abilities: “Afraid we’ll know”, “I can kill you with my brain”)
14. The Message [#10 on the DVD & Netflix] (reference to River’s psi abilities: “Mind-reading genius that can’t eat an ice planet”; reference to theft of the Lassiter in Trash)
Aside from the pilot Serenity, this is the same order as that of the production codes in Wikipedia’s List of Firefly episodes. (Related to the production order: In interviews of Greg Edmonson [the composer of the incidental music for the series], he mentions that the music for the funeral scene in The Message was intended as a goodbye for the show, since it was the last episode being worked on.)
On the other hand, Objects in Space has a more satisfying finish than the other episodes, so I conjecture that for the DVD release (when the show had been cancelled and the movie not yet planned), it was chosen to end the series.