--- a/browser/components/sessionstore/test/browser_354894_perwindowpb.js
+++ b/browser/components/sessionstore/test/browser_354894_perwindowpb.js
@@ -136,17 +136,17 @@ let setupTest = async function(options, 
 
   for (let o in observing) {
     Services.obs.addObserver(observer, o);
   }
 
   let private = options.private || false;
   let newWin = await promiseNewWindowLoaded({ private });
 
-  injectTestTabs(newWin);
+  await injectTestTabs(newWin);
 
   await testFunction(newWin, observing);
 
   let count = getBrowserWindowsCount();
   is(count.open, 0, "Got right number of open windows");
   is(count.winstates, 1, "Got right number of stored window states");
 
   for (let o in observing) {
@@ -158,19 +158,19 @@ let setupTest = async function(options, 
 
 /**
  * Loads a TEST_URLS into a browser window.
  *
  * @param win (Window)
  *        The browser window to load the tabs in
  */
 function injectTestTabs(win) {
-  TEST_URLS.forEach(function(url) {
-    win.gBrowser.addTab(url);
-  });
+  let promises = TEST_URLS.map(url => win.gBrowser.addTab(url))
+                          .map(tab => BrowserTestUtils.browserLoaded(tab.linkedBrowser));
+  return Promise.all(promises);
 }
 
 /**
  * Attempts to close a window via BrowserTryToCloseWindow so that
  * we get the browser-lastwindow-close-requested and
  * browser-lastwindow-close-granted observer notifications.
  *
  * @param win (Window)