diff --git a/browser/base/content/browser.js b/browser/base/content/browser.js
index 13872273e06..d837e723ac1 100644
--- a/browser/base/content/browser.js
+++ b/browser/base/content/browser.js
@@ -1563,49 +1563,6 @@ function delayedStartup(isLoadingBlank, mustLoadSidebar) {
   gHomeButton.updateTooltip(homeButton);
   gHomeButton.updatePersonalToolbarStyle(homeButton);
 
-#ifdef HAVE_SHELL_SERVICE
-  // Perform default browser checking (after window opens).
-  var shell = getShellService();
-  if (shell) {
-#ifdef DEBUG
-    var shouldCheck = false;
-#else
-    var shouldCheck = shell.shouldCheckDefaultBrowser;
-#endif
-    var willRecoverSession = false;
-    try {
-      var ss = Cc["@mozilla.org/browser/sessionstartup;1"].
-               getService(Ci.nsISessionStartup);
-      willRecoverSession =
-        (ss.sessionType == Ci.nsISessionStartup.RECOVER_SESSION);
-    }
-    catch (ex) { /* never mind; suppose SessionStore is broken */ }
-    if (shouldCheck && !shell.isDefaultBrowser(true) && !willRecoverSession) {
-      // Delay the set-default-browser prompt so it doesn't block
-      // initialisation of the session store service.
-      setTimeout(function () {
-        var brandBundle = document.getElementById("bundle_brand");
-        var shellBundle = document.getElementById("bundle_shell");
-
-        var brandShortName = brandBundle.getString("brandShortName");
-        var promptTitle = shellBundle.getString("setDefaultBrowserTitle");
-        var promptMessage = shellBundle.getFormattedString("setDefaultBrowserMessage",
-                                                           [brandShortName]);
-        var checkboxLabel = shellBundle.getFormattedString("setDefaultBrowserDontAsk",
-                                                           [brandShortName]);
-        var checkEveryTime = { value: shouldCheck };
-        var ps = Services.prompt;
-        var rv = ps.confirmEx(window, promptTitle, promptMessage,
-                              ps.STD_YES_NO_BUTTONS,
-                              null, null, null, checkboxLabel, checkEveryTime);
-        if (rv == 0)
-          shell.setDefaultBrowser(true, false);
-        shell.shouldCheckDefaultBrowser = checkEveryTime.value;
-      }, 0);
-    }
-  }
-#endif
-
   // BiDi UI
   gBidiUI = isBidiEnabled();
   if (gBidiUI) {
diff --git a/browser/components/nsBrowserGlue.js b/browser/components/nsBrowserGlue.js
index d77532fd9ad..18117ba39c9 100644
--- a/browser/components/nsBrowserGlue.js
+++ b/browser/components/nsBrowserGlue.js
@@ -426,6 +426,49 @@ BrowserGlue.prototype = {
 
     let keywordURLUserSet = Services.prefs.prefHasUserValue("keyword.URL");
     Services.telemetry.getHistogramById("FX_KEYWORD_URL_USERSET").add(keywordURLUserSet);
+
+    // Perform default browser checking.
+    var shell;
+    try {
+      shell = Components.classes["@mozilla.org/browser/shell-service;1"]
+        .getService(Components.interfaces.nsIShellService);
+    } catch (e) { }
+    if (shell) {
+#ifdef DEBUG
+      var shouldCheck = false;
+#else
+      var shouldCheck = shell.shouldCheckDefaultBrowser;
+#endif
+      var willRecoverSession = false;
+      try {
+        var ss = Cc["@mozilla.org/browser/sessionstartup;1"].
+                 getService(Ci.nsISessionStartup);
+        willRecoverSession =
+          (ss.sessionType == Ci.nsISessionStartup.RECOVER_SESSION);
+      }
+      catch (ex) { /* never mind; suppose SessionStore is broken */ }
+      if (shouldCheck && !shell.isDefaultBrowser(true) && !willRecoverSession) {
+        Services.tm.mainThread.dispatch(function() {
+          var brandBundle = win.document.getElementById("bundle_brand");
+          var shellBundle = win.document.getElementById("bundle_shell");
+  
+          var brandShortName = brandBundle.getString("brandShortName");
+          var promptTitle = shellBundle.getString("setDefaultBrowserTitle");
+          var promptMessage = shellBundle.getFormattedString("setDefaultBrowserMessage",
+                                                             [brandShortName]);
+          var checkboxLabel = shellBundle.getFormattedString("setDefaultBrowserDontAsk",
+                                                             [brandShortName]);
+          var checkEveryTime = { value: shouldCheck };
+          var ps = Services.prompt;
+          var rv = ps.confirmEx(win, promptTitle, promptMessage,
+                                ps.STD_YES_NO_BUTTONS,
+                                null, null, null, checkboxLabel, checkEveryTime);
+          if (rv == 0)
+            shell.setDefaultBrowser(true, false);
+          shell.shouldCheckDefaultBrowser = checkEveryTime.value;
+        }, Ci.nsIThread.DISPATCH_NORMAL);
+      }
+    }
   },
 
   _onQuitRequest: function BG__onQuitRequest(aCancelQuit, aQuitType) {