Matt Groening has specifically said that in the world of The Simpsons that Futurama is a TV show set in the future. In the world of Futurama The Simpsons is a 1,000 year old TV show. This was displayed in the Futurama episode a "A Big Ball of Garbage" when Fry and Bender come upon a pile of old Bart Simpsons Dolls.
This was addressed in their first crossover "The Simpsons Futurama Crossover Crisis" Comic Book by referring back to the Futurama Episode "The Day the Earth Stood Stupid" we see that the Brain Spawn can transport people into the world of a book. But in this episode it is completely disregarded and assumed that The Simpsons and Futurama inhabit the same universe/timeline.
This was addressed in their first crossover "The Simpsons Futurama Crossover Crisis" Comic Book by referring back to the Futurama Episode "The Day the Earth Stood Stupid" we see that the Brain Spawn can transport people into the world of a book. But in this episode it is completely disregarded and assumed that The Simpsons and Futurama inhabit the same universe/timeline.