--- a/dom/imptests/parseFailures.py
+++ b/dom/imptests/parseFailures.py
@@ -28,17 +28,17 @@ def ensuredir(path):
 
 def dumpFailures(lines):
     files = []
     for url, objstr in lines:
         if objstr == '{}\n':
             continue
 
         # Avoid overly large diffs.
-        if url.contains("/editing/"):
+        if '/editing/' in url:
             sep = ':'
         else:
             sep = ': '
 
         jsonpath = 'failures/' + url + '.json'
         files.append(jsonpath)
         ensuredir(jsonpath)
         obj = json.loads(objstr, object_pairs_hook=collections.OrderedDict)

--- a/xpfe/appshell/src/nsWebShellWindow.cpp
+++ b/xpfe/appshell/src/nsWebShellWindow.cpp
@@ -199,27 +199,28 @@ nsresult nsWebShellWindow::Initialize(ns
 
   // Eagerly create an about:blank content viewer with the right principal here,
   // rather than letting it happening in the upcoming call to
   // SetInitialPrincipalToSubject. This avoids creating the about:blank document
   // and then blowing it away with a second one, which can cause problems for the
   // top-level chrome window case. See bug 789773.
   nsCOMPtr<nsIScriptSecurityManager> ssm =
     do_GetService(NS_SCRIPTSECURITYMANAGER_CONTRACTID, &rv);
-  MOZ_ASSERT(NS_SUCCEEDED(rv) && ssm);
-  nsCOMPtr<nsIPrincipal> principal;
-  ssm->GetSubjectPrincipal(getter_AddRefs(principal));
-  if (!principal) {
-    ssm->GetSystemPrincipal(getter_AddRefs(principal));
+  if (ssm) { // Sometimes this happens really early  See bug 793370.
+    nsCOMPtr<nsIPrincipal> principal;
+    ssm->GetSubjectPrincipal(getter_AddRefs(principal));
+    if (!principal) {
+      ssm->GetSystemPrincipal(getter_AddRefs(principal));
+    }
+    rv = mDocShell->CreateAboutBlankContentViewer(principal);
+    NS_ENSURE_SUCCESS(rv, rv);
+    nsCOMPtr<nsIDocument> doc = do_GetInterface(mDocShell);
+    NS_ENSURE_TRUE(!!doc, NS_ERROR_FAILURE);
+    doc->SetIsInitialDocument(true);
   }
-  rv = mDocShell->CreateAboutBlankContentViewer(principal);
-  NS_ENSURE_SUCCESS(rv, rv);
-  nsCOMPtr<nsIDocument> doc = do_GetInterface(mDocShell);
-  NS_ENSURE_TRUE(!!doc, NS_ERROR_FAILURE);
-  doc->SetIsInitialDocument(true);
 
   if (nullptr != aUrl)  {
     nsCString tmpStr;
 
     rv = aUrl->GetSpec(tmpStr);
     if (NS_FAILED(rv)) return rv;
 
     NS_ConvertUTF8toUTF16 urlString(tmpStr);