At this point, to enable the bridge, you have to execute the following commands (having the shrewdness to replace the names of the two network interfaces fxp0 and xl0 with your own ones):

# sysctl net.link.ether.bridge.config=fxp0:0,xl0:0
# sysctl net.link.ether.bridge.ipfw=1
# sysctl net.link.ether.bridge.enable=1

The first row specifies which interfaces should be activated by the bridge, the second one will enable the firewall on the bridge and finally the third one will enable the bridge.

At this point you should be able to insert the machine between two sets of hosts without compromising any communication abilities between them. If so, the next step is to add the net.link.ether.bridge.[blah]=[blah] portions of these rows to the /etc/sysctl.conf file, in order to have them execute at startup.