That's funny cuz I was wondering the same thing. I was just given three of the biggest mulberry trees I've ever seen around these parts, all cut up firewood size and ready to haul away. I just split about half of it, so green the rounds were sending up suckers with leaves yet.
It split super nice as green as it was, nice clean splits that popped right apart almost as soon as the splitter wedge entered it.
The wood is pretty colorful too. All yellow with red streaking inside. And as the wood exposed to air dries, it turns as brown as walnut.
I didn't ask questions about whether it was any good or not. Free wood is free heat.