I've worked for two different Senators, and I've worked as a consultant for several members of the House of Representatives. And I've learned from that experience that preprinted postcards are about the worst possible thing you can send to get your point across. Every member of Congress gets thousands of postcards a year, and they're largely ignored. They get thrown into a big basket, and every so often some junior staffer will be assigned to go through and count them before throwing them away.
Actual letters in actual envelopes are far better, because someone has to actually do something to find out what a letter is about. With a postcard, once you've read the first one, all the subsequent ones can be effectively ignored, since they're obviously all the same. But with a letter, especially a letter with a typed address, has to be opened and read. That makes an impression.
(The only thing less effective than a pre-printed postcard is an online petition. FYI.)
So here's what you should do if you want to urge Fox to not cancel Firefly. Go to the aforementioned Firefly Immediate Assistance site. Copy down the address of Fox Television, and download one of the postcards. Then take the postcard and retype the text in the word processor of your choice. Print out the letter, and an envelope addressed to Fox. Mail the letter, and throw away the postcard. Trust me, it will be much more effective.