It kept happening when I opened Sage plots in Firefox. The recommendation was
to set StartupNotify to false in the correct .desktop file. The issue
was finding the right one. It wasn't anything for Firefox specifically but in
/usr/share/applications/exo-web-browser.desktop, which invokes Firefox.
Had I not figured this out I could have kept using killall xfwm4 as it
automatically respawns.