According to the marketing materials, the proprietary Firewalla WIFI-SD USB dongle/antenna only works with their (excellent) hardware firewalls. But I needed an external WIFI antenna for a Raspberry Pi, so I inserted it into a USB port to see if it would work, and yeah it does. The downside is that the Pi only seems to have drivers to use it on the 2.4 GHz band, while the antenna is supposed to be a WIFI 5 device.

It is apparently based on the dual-band RTL8821CU chipset (802.11ac). Probably you could get it working on 5 Ghz if you were prepared to tinker with drivers but I'm told this chipset is notoriously difficult to work with, and kernel updates would probably keep breaking it, so I'll pass. Anyway, if you login to a Firewalla box via SSH the default username is 'pi', so you can probably guess what the Firewalla software was developed on, and why this works.