Like Otis, my first letter would be an enthusiastic and zealous defence of Futurama. I'll write what I love about the show, the high quality of animation, the great characters, general feelings towards the show in the fan community etc. I'll put in a few words about the huge worldwide appeal Futurama has, and how many fans the show has around the globe.
But first and foremost, I'll be polite. Nothing puts off big TV execs, than negativity about their decisions. And we must keep in mind that FOX execs is way out of touch with reality, with living high up in an ivory tower and all.
If that doesn't work, and they cancel Futurama, my next letter would probably be of the sort, that would led to a restraining order, or criminal trial.
And welcome back, Mark.