--- a/testing/marionette/harness/marionette/tests/unit/test_capabilities.py
+++ b/testing/marionette/harness/marionette/tests/unit/test_capabilities.py
@@ -33,16 +33,20 @@ class TestCapabilities(MarionetteTestCas
         self.assertEqual(self.caps["specificationLevel"], 0)
 
     def test_supported_features(self):
         self.assertIn("rotatable", self.caps)
         self.assertIn("acceptSslCerts", self.caps)
 
         self.assertFalse(self.caps["acceptSslCerts"])
 
+    def test_additional_capabilities(self):
+        self.assertIn("processId", self.caps)
+        self.assertEqual(self.caps["processId"], self.appinfo["processID"])
+
     def test_we_can_pass_in_capabilities_on_session_start(self):
         self.marionette.delete_session()
         capabilities = {"desiredCapabilities": {"somethingAwesome": "cake"}}
         self.marionette.start_session(capabilities)
         caps = self.marionette.session_capabilities
         self.assertIn("somethingAwesome", caps)
 
     def test_set_specification_level(self):