Yeah, avengers and dark night better than fight club? C'mon.
Er, how, exactly?
Firefly doesn't have that sort of psychological depth, but it does have a lot of moral ambiguity, something that's virtually non-existant in CK. That's something that would have gone a lot deeper had the show gotten another two or three seasons, but they never got that chance. It also has some of the most clever writing of any TV show, up there with only Arrested Development really. Cleverness might not be as prized by critics as authentic character writing, but it's still at the top of its form nonetheless. The main downside of the show was that it was already starting to get a bit formulaic toward the end of season one when they canceled it. In any event, I don't think you could ever put Firefly above Citizen Kane, but certainly I think it pushed the boundaries of its genre in a way that is rare.
The Avengers, on the other hand, has virtually no redeeming value whatsoever except for Joss Whedon's clever writing during the second half and the amazing effects during the NYC fight scene.
Citizen Kane gets a lot of rightful attention for being historic and groundbreaking in a number of ways, especially by people who have never seen it. Unfortunately, despite often being conflated, "groundbreaking" and "good" are not really the same thing. Now, don't get me wrong, Citizen Kane is a fine film and still worth watching today. But divorce it from its historic role and consider modern audiences. Think about what it would be like if released today. At best it would be critically-acclaimed and lauded by a small audience without ever seeing mainstream appeal, much as Firefly has.
Now, granted, I haven't gone a single shred toward showing that Firefly is actually any better than Citizen Kane though I have gone some way to putting them on roughly the same level. And Firefly has more explosions and space zombies making it more watchable for many people today.
Is Firefly really better than Citizen Kane? It will never be widely thought so by critics but I can definitely see an argument where it is better for modern audiences and I really think that's what matters at the end of the day.