Tracking the thread through https://bugzilla.mozilla.org/show_bug.cgi?id=1207700 and https://bugzilla.mozilla.org/show_bug.cgi?id=1170342 (and ultimately https://bugs.freedesktop.org/show_bug.cgi?id=93539 ) it appears that when XI2 is used, focus-out events were not sent in rare cases under old versions of Xorg. If you're running modern software there should be no downside.