--- a/browser/components/preferences/in-content/subdialogs.js
+++ b/browser/components/preferences/in-content/subdialogs.js
@@ -45,16 +45,17 @@ let gSubDialog = {
     let features = (!!aFeatures ? aFeatures + "," : "") + "resizable,dialog=no,centerscreen";
     let dialog = window.openDialog(aURL, "dialogFrame", features, aParams);
     if (aClosingCallback) {
       this._closingCallback = aClosingCallback.bind(dialog);
     }
 
     this._closingEvent = null;
     this._isClosing = false;
+    this._openedURL = aURL;
 
     features = features.replace(/,/g, "&");
     let featureParams = new URLSearchParams(features.toLowerCase());
     this._box.setAttribute("resizable", featureParams.has("resizable") &&
                                         featureParams.get("resizable") != "no" &&
                                         featureParams.get("resizable") != "0");
     return dialog;
   },
@@ -119,17 +120,17 @@ let gSubDialog = {
         this._onParentWinFocus(aEvent);
         break;
     }
   },
 
   /* Private methods */
 
   _onUnload: function(aEvent) {
-    if (aEvent.target.location.href != "about:blank") {
+    if (aEvent.target.location.href == this._openedURL) {
       this.close(this._closingEvent);
     }
   },
 
   _onContentLoaded: function(aEvent) {
     if (aEvent.target != this._frame || aEvent.target.contentWindow.location == "about:blank")
       return;