From https://phabricator.services.mozilla.com/D14911#inline-85520:

Maybe we should just handle pagehide in the handleEvent method? Or is there a special reason to do it this way?

And:

This is just how XBL was doing it: https://searchfox.org/mozilla-central/rev/76fe4bb385348d3f45bbebcf69ba8c7283dfcec7/toolkit/content/widgets/browser.xml#1102. I suspect this could be done in handleEvent, but I'll spin that off into a follow up if you don't mind.