According to Molinism, what is the meaning of a counterfactual of freedom? If I understand correctly, Molinists consider it important that counterfactuals of freedom ("If person P experienced circumstances C, she would perform action A") are meaningful. In fact, if I understand correctly, the crux of Molinism is the doctrine that the truth or falsity of a counterfactual of freedom is logically prior to Creation.

But I'm struggling to understand what a counterfactual of freedom even means from a Molinist perspective. It seems clear that the circumstances C do not logically entail the action A; we can coherently imagine P experiencing circumstances C but nevertheless refraining from A. But if I understand correctly, Molinists endorse a libertarian notion of free will, in the sense that the circumstances C do not cause the action A, either.

So in what sense is A the unique action that P "would" perform, were she to experience circumstances C?
The confusion may arise from the way you've stated the view.  A clearer version might be:

"If person P experienced circumstances C, she would freely choose to perform action A."

This doesn't mean that P must choose A, it means that P will freely choose A.  If P were to freely choose not-A, then God would know something different from eternity, namely P will choose not-A, given C.

An example: if you offered your friend the opportunity to freely choose to stick his hand in a pot of boiling water for the fun of it, you 
"know" (really "assume", since you're not omniscient) that he would never do so.  The Molinist view is similar, but stronger. Since God knows all things infallibly, he knows with absolute certainty what you would freely do in all possible circumstances, not just the extreme, boiling water contrivances.

An excellent (and short) book on this topic is The Only Wise God:  The Compatibility of Divine Foreknowledge and Human Freedom  by Dr. William Lane Craig.