I got 2.5GbE PCI-e card, but no drivers present in pfSense installation. To list devices:

pciconf -lv

There is this Realtek network card:

re0@pci0:4:0:0:	class=0x020000 rev=0x05 hdr=0x00 vendor=0x10ec device=0x8125 subvendor=0x10ec subdevice=0x0123
    vendor     = 'Realtek Semiconductor Co., Ltd.'
    device     = 'RTL8125 2.5GbE Controller'
    class      = network
    subclass   = ethernet

But there is no driver enabled:

pkg info | grep driver
kldstat

I installed realtek-re-kmod-198.00_3 package. It was few months ago, but I am pretty sure that is was from pkg and not by downloading it from some repository manually. Finally add to /boot/loader.conf:

if_re_load="YES"
if_re_name="/boot/modules/if_re.ko"

So you can either download from FreeBSD repository or:

pkg search realtek