In my opinion, the three shows show a progression in how Joss presents his existentialism. Buffy is centered around how Joss gives his life meaning. As Marti Noxon said in an interview with the CBC Program "Ideas"
I think that he does feel like it's sort of a meaningless void, and what matters is the struggle to find the good. And the relationships you build with people while you struggle. And in some ways you'll never find it, but the quest and the questors, and the people that you find, who are not necessarily your family, are the only thing that lends the journey meaning. I think that is his major theme.
That is a very good summation of Buffy. I doubt anyone can argue with that (well I'm sure they could, but they'd be wrong). Not all Buffy fans like Angel, even though it is the spin-off. They have any number of complaints. Most of those complaints can be traced back to one thing, Angel isn't just about what I've quoted above. It makes some strong statements about existentialism on a metaphorical level. (If anyone is interested in this, I have several essays, I'd be happy to either post or link).
So we go from, just giving life meaning through the journey and family/friends, to dealing with the existentialist's view of the universe illustrated on a metaphorical level, to Firefly.
Anyone who has watched the commentary to "Objects in Space" cannot deny that Firefly is written from an existentialist's perspective. I'm going sum up existentialism with one sentence: It is the universe laid bare. It isn't something you can intellectually comprehend. You can understand it, but not comprehend, to paraphrase River. You can't comprehend, because the belief itself, the subject of "Jaynestown," is required to comprehend comprehension itself.
Comprehension is understanding the significance of something. In existentialism there is no inherent significance to anything. It is us that give things significance, imbue them with meaning. That is the task set before Mal and all the characters. For whatever reason, their life has been turned upside down. That stripped life bare, took away what they thought gave their life meaning. Firefly is about giving something meaning again.
Most of my Buffy friends don't like Firefly or at least are head over backwards in love with it like I am. Even though they love all things Joss, this wasn't something that could reach into them like his other shows. For me, this up front, in your face raw existentialism was something that hit me in all cylinders.
Take a piece of paper. Now rip it. Tear it however you want. Is that now damaged? It has changed. The word damaged implies that it is supposed to be something particular that it is no longer. The opening of Mal's eyes and River's new mental state are only damage if you assume that humans are supposed to be a certain way. That is not an assumption an existentialist can make.
Firefly constantly caused me to challenge my assumptions about the characters and about myself. That imbued the show with meaning for me. A bold existentialist statement...who could ask for anything more.