--- a/dom/webidl/Navigator.webidl
+++ b/dom/webidl/Navigator.webidl
@@ -337,16 +337,19 @@ partial interface Navigator {
   [Pref="dom.gamepad.test.enabled"]
   GamepadServiceTest requestGamepadServiceTest();
 };
 #endif // MOZ_GAMEPAD
 
 partial interface Navigator {
   [Throws, Pref="dom.vr.enabled"]
   Promise<sequence<VRDisplay>> getVRDisplays();
+  // TODO: Use FrozenArray once available. (Bug 1236777)
+  [Frozen, Cached, Pure, Pref="dom.vr.enabled"]
+  readonly attribute sequence<VRDisplay> activeVRDisplays;
 };
 
 #ifdef MOZ_B2G_BT
 partial interface Navigator {
   [Throws, ChromeOnly, UnsafeInPrerendering]
   readonly attribute BluetoothManager mozBluetooth;
 };
 #endif // MOZ_B2G_BT