Hi,
Bridge interface is supported on the SSG Series of ScreenOS. Bridge Groups are new to ScreenOS starting with version 6.0. on the SSG firewall family. These represent a logical Layer 2 switch within the firewall. You can configure any port on an SSG5/20 into a Bridge Group; on the SSG140/300/500 family, only ports added via Universal Port Interface Modules, uPIMs, can be in a Bridge Group and the group cannot span uPIM modules. This is documented on the Juniper Support Site Knowledge Base within KB article number 10747, located at http://kb.juniper.net/KB10747.
ISG series do not support this concept and the reason must be the difference in the architecture.
Hope this helps.
Regards,
Pradeep.