Thoughts precede neuron firing or vise-verse?
I think the only way to interpret these kinds of questions biologically is to assume that "thought" means "the perception of a thought occurring." The perception of a thought, like all perception, arises from neuronal firing.

The interesting biological (rather than philosophical) questions arise when we look at the relationship between the neural firing that gives rise to the perception of intent to act and neural firing that carries out the "intended" actions.

This paper is relevant. The authors are trying to argue that the initiation of a motor action comes before the perception of intent for that action. I'm not a cognitive neuroscience person, though, so I'm not very familiar with their techniques and personally have a hard time determining if they draw valid conclusions.