Alright. I spent a little while figuring this out.
It turns out that Mellanox has basically dropped all support for this NIC. However, not all is lost. You can still use the legacy versions for ConnectX-3 to actually utilize the NIC (or so says the release notes for the driver I downloaded). You can find the driver webpage here, and the actual driver download right here. It's under
ConnectX-3 -> Archive Versions -> 2.1 -> FreeBSD -> All -> All on the actual driver site.
Fortunately it comes with instructions on how to install the driver. Since pfSense doesn't come with suitable environment for compiling, you'll have to set up a separate machine to compile. I installed FreeBSD 10.3 (with source code; it's an option in the installer) in a virtual machine, which is the same release that my version of pfSense is using. Be sure to use the same release of FreeBSD as the one used in your pfSense box. It might cause problems otherwise.
I installed gcc using
pkg install gcc. Unfortunately, at this point, I wasn't able to actually use said drivers I downloaded from the Mellanox site. However, FreeBSD does still have support for the drivers, they're just not enabled by default. If you find you get an error when building the drivers, you can find the two modules you need to compile in
/usr/src/sys/modules. There will be two directories of interest,
mlx4. There will be a
Makefile in both of these directories. Do the usual
make && make install procedure that you'd do with any other driver. It'll drop the modules you need in
/boot/kernel. The module names are
mlxen.ko. Copy both of these modules to your pfSense box (into the same directory:
/boot/kernel). Afterwards, you can add these two lines to your
Save the file and restart your pfSense machine.
For me, the NIC now shows up and allows itself to be configured as a LAN interface.