In general ZFS may try to use more RAM than other filesystems (like UFS) but a lot depends on the actual usage patterns. Reads cause ARC (read cache) to increase which increases RAM useage, writes wind up in a different location, simplistically "transaction groups". Writes are held until either a timer expires (nominally 30secs) or a lot of items need to be written (not sure but on the order of maybe 4K or 4M).
A firewall would be mostly writes so I'd expect it to go up and down if you are logging a lot. Not sure if pfSense is creating a RAM disk for anything, but that would also increase useage.
The cached memory could be ZFS buffers, could also be network buffers.