It's true. I got massively absorbed in it and did the whole series and movie in one week. Now I'm bereft. I'm going to soak up all the peripheral stuff I can, then watch the series again.
I recommend
Finding Serenity, a collection of essays about Firefly. For Christmas, I received volumes one and two of
Firefly: The Official Companion, were are filled with unedited shooting scripts, interviews with Joss Whedon and the actors, lots of rare and previously unpublished pictures, and lots more. I've only just started perusing them, but great material for a serious Firefly fan.
Then, I'll watch all 10 seasons of SG-1 again, which will take MONTHS, then do Firefly again and maybe during that time, another brilliant piece of SciFi will surface.
I'd like to second the recommendations for the new Battlestar Galactica and Buffy. Battlestar Galactica is also a well-written, highly character-driven sci-fi drama, focusing on a rag-tag band of underdogs, so it should appeal to fans of Firefly.
Although Buffy is comic/horror/adventure, Joss Whedon is a genius, and if you like Firefly, you'll probably like his his other works as well. Firefly was actually my introduction to Joss, but after that, we devoured all seven seasons of Buffy in just a couple of months. Angel is on our list, but we haven't gotten around to it yet.
You might also want to check out Babylon 5 and Farscape. Although they aren't quite as good as Firefly and Battlestar Galactica, they are still quite good, and worthy of the attention of a sci-fi fan.