I would expect that if you have two identical motherboards with the same BIOS version, you number ports on multi port cards the same way (e.g. always increasing port number as you go away from the mother board) and you move the cards from one motherboard to another such that the card in slot "n" goes to slot "n" in the new motherboard then the FreeBSD device name of a port on a card would be unchanged.
As I think I explained earlier, if you add or remove cards the unit numbers of existing ports is likely to change.
If you have a number of identical systems (same motherboard, same BIOs etc) and you configure them with the same types of cards in the slots (e.g. all mother boards have a 2 port PCI card in slot 2 etc) then I would expect all ports in the same position to have same name across all systems (e.g. port 3 in slot 2 is called em4 on all systems). However this is one caveat: As best I know ALL Intel GigE PCI chips are handled by the em driver. I believe SOME Intel GigE PCIe chips are handled by the em driver and others by the igb driver. Its possible some PCIe GigE cards have a different chips than other PCIe cards with the same number of ports so it may be a particular PCIe 2 port card provides em <n>and em <n+1>while another PCIe 2 port card might provide igb <n>and igb <n+1>and that can affect the naming of ports on other cards.
If you think you have found a configuration that violates what I have written I'm happy to take a look but I would need at least the output of pfSense shell command pciconf -l -v from both systems and a very clear indication of the difference (e.g. motherboard LAN port is em0 in the first configuration and em9 in the second).</n+1></n></n+1></n>