When you talk about a switching device you mean a L3 switch doing routing?
Your transit network would be an interface on pfsense in its own network, and then another interface on your mx100 which is a firewall/router.. While it might have "switch" ports on it its an actual router/firewall just like pfsense.
The transit network would be from an interface on your pfsense router to an interface on your mx100. How that gets switch would be at L2.. So you could either have a connection going from pfsense directly to the mx100 or over switch (with nothing else on it dumb switch) or over a L2 switch via a vlan (smart/managed switch).