Sounds great! It seems out of all the shows I introduce to people, Firefly is the one that the most people seem to like. I even know quite a few sci-fi haters who warm to the show. I think people really connect with it's humor and fun lighthearted atmosphere.
A television marathon can be great with the right crowd. I've hosted an annual event for several years when the new season of 24 is released (season 8 came out today, so we are looking for a good date now). Just be sure to stock up on the drinks and foods of your choice.