Bridges are simple, protocol-dependent networking devices that are used to connect two or more homogeneous LANs to form an extended LAN.
A bridge does not change the contents of the frame being transmitted but acts as a relay.
A gateway is designed to reduce the problems of interfacing any combination of local networks that employ different level protocols or local and long-haul networks.
A router connects two networks or network segments and may use IP to route messages.
Firewalls are methods of protecting a network against security threats from other systems or networks by centralizing and controlling access to the protected network segment.
Source: HARRIS, Shon, All-In-One CISSP Certification Exam Guide, McGraw-Hill/Osborne, 2002, chapter 7: Telecommunications and Network Security (page 397).