--- a/services/sync/tests/unit/test_storage_sync_engine.js
+++ b/services/sync/tests/unit/test_storage_sync_engine.js
@@ -36,11 +36,29 @@ add_task(function* test_calling_sync_cal
   try {
     yield engine.sync();
   } finally {
     StorageSyncEngine.prototype._sync = oldSync;
   }
   equal(syncMock.calls.length, 1);
 });
 
+add_task(function* test_calling_sync_calls_ext_storage_sync() {
+  const extensionId = "my-extension";
+  let oldSync = ExtensionStorageSync.syncAll;
+  let syncMock = ExtensionStorageSync.syncAll = mock({returns: true});
+  try {
+    yield* withSyncContext(function* (context) {
+      // Set something so that everyone knows that we're using storage.sync
+      yield ExtensionStorageSync.set(extensionId, {"a": "b"}, context);
+
+      yield engine._sync();
+    });
+  } finally {
+    ExtensionStorageSync.syncAll = oldSync;
+  }
+  do_check_true(syncMock.calls.length >= 1);
+});
+
+
 function run_test() {
   run_next_test();
 }