I'm not sure I'd call Firefly "hard" sci-fi. But it is much more down-to-earth (so to speak) than most TV sci-fi. There's no sound in space, no faster-than-light travel, no aliens etc... Joss actually wanted to avoid the 'artificial gravity' conceit, but it was just too demanding budget-wise. The most realistic aspect, and what makes the show near and dear to my heart, is that it's about "real" people facing real challenges. In Joss's words, the show "isn't about the people who made history, it's about the people history stepped on".
The original idea for the series was inspired by the plight of Southern soldiers dealing with the realities of the post civil-war American South. Mal, the captain of the ship, is a relatively dark character, bitter and jaded after coming out on the losing side of a system-wide civil war. Now, he rides the fringes of civilized society (and the fringes of the law) ferrying freight to the outer planets on the frontier.
The style of the story-telling is actually more properly considered "western", or even "family drama" than traditional science fiction, as it's more about Mal's struggle to keep his crew together and out of reach of the "feds" (yes, they actually call them "feds"
). Arguably the best episode in the series, "Out of Gas" is about a mechanical breakdown that leaves the crew stranded in open space. That one really got to me. I've never fought an alien empire to save the galaxy from ultimate evil. But I have been stranded on the side of the road in the middle of nowhere, late at night, out of gas, with no cell-phone and my kids in the back seat - wondering whether the approaching headlights in the distance were friend or foe....
As to those of you doubting the ultimate goodness of Firefly, well, everyone is entitled to their own ill-informed, uncultured opinion.
But Firefly is good stuff.