I am not sure what you mean by bridge mode ? What do you understand it to mean? Need you to help me understand :-)
I have heard the term used when it means 'straight modem-only mode' and with a modem-router, the device stops being a router and becomes a straight modem, and needs to be connected to a router via an ethernet cable. The router then performs routing, firewalling, NAT, possibly is a wireless access point too, and does a host of other things. The modem is now just that, it just makes a link between the router and the dsl internet connection but is otherwise completely dumb.