Bridge Mode on an ISP-owned modem (e.g., Fiber modem, Cable Modem, DSL modem) turns a device into Layer 2 bridge (e.g., Fiber-to-Ethernet). Most modems today come packaged with routing, firewall, and wireless capabilities. But when you configure your modem for Bridge mode, you assign the perimeter firewall functionality to a downstream device: your trusted perimeter firewall (e.g., pfSense).
When you're using Sovereign Stack, you want to turn all that extra stuff off on the modem and convert your "router/firewall" into a bridge (e.g., fiber-to-ethernet). The responsibility of the ISP modem then is the provide an uplink to the Internet, and ultimately, a DHCP address to the firewall WAN/outside interface.