Modal: Ignore spurious focus event that Firefox fires at document when switching back to its tab

Fixes #18365
Ports #18638 to v3
Refs https://bugzilla.mozilla.org/show_bug.cgi?id=1228802

js/modal.js

@@ -140,7 +140,9 @@
     $(document)
       .off('focusin.bs.modal') // guard against infinite focus loop
       .on('focusin.bs.modal', $.proxy(function (e) {
-        if (this.$element[0] !== e.target && !this.$element.has(e.target).length) {
+        if (document !== event.target &&
+            this.$element[0] !== e.target &&
+            !this.$element.has(e.target).length) {
           this.$element.trigger('focus')
         }
       }, this))