diff --git a/platform/firefox/vapi-background.js b/platform/firefox/vapi-background.js
index 7a85730ff..40157849f 100644
--- a/platform/firefox/vapi-background.js
+++ b/platform/firefox/vapi-background.js
@@ -680,6 +680,12 @@ var winWatcher = (function() {
         if ( !win || windowToIdMap.delete(win) !== true ) {
             return;
         }
+        // https://github.com/uBlockOrigin/uAssets/issues/567
+        //   We need to cleanup if and only if the window being closed is
+        //   the actual top window.
+        if ( win.gBrowser && win.gBrowser.ownerGlobal !== win ) {
+            return;
+        }
         if ( typeof api.onCloseWindow === 'function' ) {
             api.onCloseWindow(win);
         }