None. There's nothing that good. of course you can try Battlestar Galactica, Space: Above and Beyond, and the animated Cowboy BeBop.

In terms of space exploration Battlestar Galactica is the only thing that comes close to Firefly, but even though both are gritty, intelligent and funny they tick all the boxes in very different ways, both are brilliant for different reasons.

No other space explorations shows have floated my boat like those two, not even any of the Star Trek series, I would however recommend the following sci-fi TV shows:

Fringe - 21st century X-Files, but in many (not all) ways better.
Dollhouse - Joss Whedon's only post-Firefly tv project (that I'm aware of). Like all good sci-fi it's based around a single idea, but manages to brilliantly make some superb observations around it, only two series but damn did it sign off in spectacular fashion!
Game Of Thrones. I know it's not sci-fi but my gods it's just one of the greatest things ever.

It's tough to find anything that lives up to Firefly in terms of tone but along with what's been mentioned before, I'd recommend Farscape (since only one person mentioned it). Nice long story arcs that all get wrapped up in a movie at the end. Lots of humor and drama with some decent action mixed in.

Speaking of humor, drama and action, I'd also heartily recommend the Stargate series. Starting with Stargate SG-1 then alternating episodes of Stargate Atlantis when that storyline branches off. They mix characters and storylines from the two really well yet manage to keep them as their own shows. The first season of SG-1 obviously isn't the strongest, but they lay the groundwork for the entire series there so it's worth it.

After all of that and the two SG-1 movies I'd jump into Stargate Universe. Much different in tone and cinematography but it also fits nicely into the Stargate mythology and they were able to end the series on their own terms when they got cancelled by the end of the second season.