--- a/embedding/components/windowwatcher/nsWindowWatcher.cpp
+++ b/embedding/components/windowwatcher/nsWindowWatcher.cpp
@@ -449,36 +449,33 @@ nsWindowWatcher::OpenWindow2(mozIDOMWind
   return OpenWindowInternal(aParent, aUrl, aName, aFeatures,
                             aCalledFromScript, dialog,
                             aNavigate, aOpeningTab, argv,
                             aOptionalArgc >= 1 ? &aOpenerFullZoom : nullptr,
                             aResult);
 }
 
 // This static function checks if the aDocShell uses an UserContextId equal to
-// nsIScriptSecurityManager::DEFAULT_USER_CONTEXT_ID or equal to the
-// userContextId of subjectPrincipal, if not null.
+// the userContextId of subjectPrincipal, if not null.
 static bool
 CheckUserContextCompatibility(nsIDocShell* aDocShell)
 {
   MOZ_ASSERT(aDocShell);
 
   uint32_t userContextId =
     static_cast<nsDocShell*>(aDocShell)->GetOriginAttributes().mUserContextId;
 
-  if (userContextId == nsIScriptSecurityManager::DEFAULT_USER_CONTEXT_ID) {
-    return true;
-  }
-
   nsCOMPtr<nsIPrincipal> subjectPrincipal =
     nsContentUtils::GetCurrentJSContext()
       ? nsContentUtils::SubjectPrincipal() : nullptr;
 
+  // If we don't have a valid principal, probably we are in e10s mode, parent
+  // side.
   if (!subjectPrincipal) {
-    return false;
+    return true;
   }
 
   // DocShell can have UsercontextID set but loading a document with system
   // principal. In this case, we consider everything ok.
   if (nsContentUtils::IsSystemPrincipal(subjectPrincipal)) {
     return true;
   }