Straight shooting, here's how the firewood burning science (heat measured in British Thermal Units) works. The actual measure is based on how long it burns multiplied by its actual intensity. As a result, the derived heat measured (BTU's) is determined by various factors. Namely, the species of wood, how dry it is and its actual condition (physical and chemical).
Some species' of wood provide more BTU's than others. Based on common species found here, a rule of thumb (although there are exceptions) is that the denser darker types (ex: Oak, Apple, Hickory) are rated highest. However, these take longer to dry, harder to get burning and to some smell funky. Conversely, those lower ranked types (ex: Ash, soft maples, lighter birches) dry fast, easy to handle and ignite quickly.
When considering a wood's dryness, seasoning is the most important factor. Whilst processing a tree after the sap runs from the roots to periphery renders a much longer time at which the wood will be deemed "viable".
Lastly, the condition of the wood plays a crucial role. Generally speaking, Bark comprises roughly 20% of a piece of wood's surface area. Conversely, its BTU rating is piss-ant (despite the species), it creates mess and a lot of bug species' live, lay there eggs in it, and the like. The physical/chemical condition of the wood (ultimately determining BTU's) is a fabrication of the tree's overall health. For instance, a piece of the same size, bark composition and species with bug holes, punk (rot), disease, and the like will be much worse than a solid healthy equal counterpart.