Joss had proclaimed himself to be an "angry atheist existentialist" in interviews before Firefly
was even an idea. Those beliefs permeate the Buffyverse. They are just below the surface of many storylines. They don't make it to the surface, though. Even Angel season 5 is existentialist on a metaphorical level only. In Firefly
Joss doesn't say au revoir to metaphor, but his existentialism is more on the surface, especially in the final episode aired "Objects in Space."
This is an episode that is difficult enough to understand intellectually, let alone comprehend. River tells that to Mal, "She understands. She doesn't comprehend." The difference between these two words, that are often thought of as synonyms, is important. When Mal is speaking to River, she understand what he is saying. The words he is saying in that particular order make sense to her. That is one level of apprehension. What she misses is the significance of those words that is comprehension. Mal's significance isn't hers. ( Cut for spoiler to Objects in SpaceCollapse )
It's too bad that we didn't get to see River after she started coming to terms with things. It's too bad that we didn't get to watch the crew have to adapt to being confined in space with River. Both sides of the equation are fascinating.