Is fossilized tree sap used as fuel?

I believe you're referring to amber, though this is a common misconception. Amber is not actually made from tree sap, but from tree resin, a substance that acts to seal wounds in trees. When it's still in liquid form, it's used for things like varnish and adhesives. Over the course of thousands, perhaps millions, of years, it fossilizes. The most common use is in jewelry. In general, it's also admired by collectors, geologists, prehistoric archeologists, etc. Because it was once liquid, it often occurs that insects or other small creatures become trapped, freezing them in time and preserving them perfectly. Remember Jurassic Park, where the scientists are extracting DNA from a mosquito? Amber is the stone they're drilling into. Unfortunately, I don't think we'll be cloning any dinosaurs anytime soon, even if we could recover a viable sample of dinosaur DNA.