The article "Filtering Bridges" at https://www.freebsd.org/doc/en/articles/filtering-bridges/article.html contains apparently incorrect information in sections 2.2 and 4. that would surely prevent any reader from succeeding with this software. I believe the information provided is for a long ago version of FreeBSD. The correct information is in the if_bridge(4) man page in 10.2 and no doubt earlier releases. The article has a last revised date in April 2015, which misleads the reader into thinking the information is reasonably current. In section 2.2 "bridge" should be "if_bridge". In section 4 the various sysctl commands are no longer valid. The current version of the software uses ifconfig commands to control the bridge. See if_bridge(4) for details. Perhaps all that need be done is to place a note under the title that the material is obsolete and available for historical purposes only.
Scratch my suggestion to mark the article as obsolete - other than sections 2.2 and 4 the material is extremely useful and I was unable to obtain similar information anywhere else.