diff --git a/lib/types/lifecycle-state-types.js b/lib/types/lifecycle-state-types.js
--- a/lib/types/lifecycle-state-types.js
+++ b/lib/types/lifecycle-state-types.js
@@ -1,3 +1,8 @@
 // @flow
 
-export type LifecycleState = 'active' | 'inactive' | 'background';
+export type LifecycleState =
+  | 'active'
+  | 'inactive'
+  | 'background'
+  | 'extension'
+  | 'unknown';