diff --git a/browser/devtools/commandline/test/browser_cmd_screenshot.js b/browser/devtools/commandline/test/browser_cmd_screenshot.js
index f3f8463b1b88..3265fc3b15b3 100644
--- a/browser/devtools/commandline/test/browser_cmd_screenshot.js
+++ b/browser/devtools/commandline/test/browser_cmd_screenshot.js
@@ -170,9 +170,7 @@ function addWindow(windowOptions, callback) {
 
   let win = OpenBrowserWindow(windowOptions);
 
-  let onLoad = function() {
-    win.removeEventListener("load", onLoad, false);
-
+  whenDelayedStartupFinished(win, function() {
     // Would like to get rid of this executeSoon, but without it the url
     // (TEST_URI) provided in addTabWithToolbarRunTests hasn't loaded
     executeSoon(function() {
@@ -187,9 +185,7 @@ function addWindow(windowOptions, callback) {
         deferred.reject(ex);
       }
     });
-  };
-
-  win.addEventListener("load", onLoad, false);
+  });
 
   return deferred.promise;
 }
diff --git a/browser/devtools/commandline/test/head.js b/browser/devtools/commandline/test/head.js
index c7e71fea3bd6..40762de18dd1 100644
--- a/browser/devtools/commandline/test/head.js
+++ b/browser/devtools/commandline/test/head.js
@@ -10,6 +10,15 @@ let testDir = gTestPath.substr(0, gTestPath.lastIndexOf("/"));
 Services.scriptloader.loadSubScript(testDir + "/helpers.js", this);
 Services.scriptloader.loadSubScript(testDir + "/mockCommands.js", this);
 
+function whenDelayedStartupFinished(aWindow, aCallback) {
+  Services.obs.addObserver(function observer(aSubject, aTopic) {
+    if (aWindow == aSubject) {
+      Services.obs.removeObserver(observer, aTopic);
+      executeSoon(aCallback);
+    }
+  }, "browser-delayed-startup-finished", false);
+}
+
 /**
  * Force GC on shutdown, because it seems that GCLI can outrun the garbage
  * collector in some situations, which causes test failures in later tests