Onelowerlight, a “thoughtful Mormon blogger”, explains why Firefly is not good science fiction. The bottom line: the scientific flaws and the preachiness. Oh, and the sex.
So, in short, the things that (IMO) made Firefly bad sci fi were: too much sex, not balancing sex with other aspects of human relationships, failure to address the classic problems of the genre (such as FTL travel and terraforming), too much preachiness (for gay rights and for a “naturalistic” worldview, ie atheism), and clumsy treatment of the role of religion in human interactions and the big unanswerable questions. There were a lot of other things I didn’t like about Firefly in general, but these here were the big things that mad it a poor piece of sci fi.
I might also direct your attention to some of the essays in Finding Serenity.
For now: discuss.