Many fast-growing trees are lighter and lower in heat value. Poplar is about the same as cottonwood and willow, and I only burn them if we are cutting it down on the property here or if it is windthrow. I would not grow it for fuel, and it is not marketable here for timber.
Around here red alder grows almost as fast, and it is a money tree. I can get $800/MBF for it at the mill right now, even in this crappy timber market, which is higher than Doug fir right now. It is also a better firewood than poplar, stabilizes stream banks and wet areas as a tree, and makes a great charcoal and wood stove cooking fuel. It also splits fairly easilly, and smells good when burned. It also burns nice and evenly in the fireplace. We also burn a lot of windthrown alder in the OWB here. I have planted a lot of alders in the past 3 years. Only bad things about alder is that it tends to die out in sections, tends to split when felling, and leaves a lot of red colored juice on your saw when felling.