The number of neurons firing does matter, but what matters even more is how they all are connected.
I personally think once we get adequate algorithms and data structures out there, the metric will switch to numbers of lookup operations performed, and floating point operations will just be a multiplicative constant in front of the whole complexity formula.
This is all armchair speculation, but I'd be willing to be my life that a flop is well below the floor of actual computation happening from a single neuron firing.
In computers the address a value is stored at doesn't encode information. Only the value held at the address matters. Not so with neurons. How the connections form and who they are connected to represent compound operations. Like passing a value through separate preconfigured pipelines.
It seems similar to music/geometric shape. Queries by frequency filtering through the geometry of the neurons activating access to some thought-space.