FreeBSD, and thus pfSense will try to efficiently use all of your RAM. "Free" RAM is wasted RAM. Why would you want it sitting there empty?
You might find that it reports high usage, when in fact a good chunk of that may be in use for caching to speed up processing. It won't swap unless it has to do so, but it will if it really does run out of RAM. If it's using some RAM for caching, and a program needs it, it will handle that without swapping.
If you have console/ssh access, look at the first few lines of "top" and it will lay out how much RAM is being used for specific purposes.