> What does that look like, exactly? What are the steps to roll back to a prior version?

I am tracking the -stable branch in ports, which means I periodically sync my local ports tree with upstream and then build / upgrade any updated packages. Inside my /usr/ports/packages/amd64/all/ directory I have the following packages:

  firefox-esr-31.4.0.tgz
  firefox-esr-31.5.0.tgz
  firefox-esr-31.5.3.tgz
  firefox-esr-31.6.0.tgz

If I somehow decide I do not want the newest firefox-esr, I would:

pkg_delete firefox-esr-31.6.0 && pkg_add firefox-esr-31.5.3

And that's it.