Whether you watch or not, consider sending an email to firstname.lastname@example.org.
Tell them how much you love the show, and how disappointed you'd be if they cancelled it. Consider telling them if you like/dislike Joss Whedon's other shows "Buffy" and "Angel." Definitely tell them your demographics - age, gender, profession, and possibly location. Make up demographics if you don't want to give your own. Consider mentioning other Fox shows that you may also like. Maybe tell them who your favorite character is, or some aspect of the show that you especially like. It doesn't have to be long and/or complicated to make a difference.
The letter I sent last week:
I just wanted to write and tell you how much I love "Firefly." I've been a fan of "Buffy" and "Angel" for a number of years, but "Firefly" has really knocked me head over heels. I've been really impressed by the stories, production, acting, and directing.
I hope you continue to air "Firefly," because it's definitely my favorite show this year, with "24" as a close second.
If you're interested in demographics, I'm a ...
Another sample letter from a friend of mine:
Upon strong recommendation from three of my closest friends, I finally got around to watching "Firefly" for the first time tonight, and I ABSOLUTELY loved it. The action, the suspense.... Kudos to your fine network for bringing us another timelessly classic television series!