# kldload ipfw # (WARNING read on first)
But be careful the default behaviour is to block everything, you need to open the connection you need manually, the first time I tried it I lost my ssh connection.
Fortunately the machine was next to me, so I logged on using the console, and unloaded the module:
# kldunload ipfw
I never worked with kernel modules with FreeBSD, this another useful command, to list the loaded modules you can use:
# kldstat
Note: This feature is little old, since 4.6