--- a/testing/marionette/client/marionette_driver/marionette.py
+++ b/testing/marionette/client/marionette_driver/marionette.py
@@ -1240,20 +1240,20 @@ class Marionette(object):
 
         resp = self._send_message("newSession", body)
         self.session_id = resp["sessionId"]
         self.session = resp["capabilities"]
         # fallback to processId can be removed in Firefox 55
         self.process_id = self.session.get("moz:processID", self.session.get("processId"))
 
         if self.instance is not None:
-            self.handled_instance_process_id = self.instance.runner.process_handler.pid
+            self.instance_pid = self.instance.runner.process_handler.pid
 
-            if self.handled_instance_process_id != self.process_id:
-                raise errors.MarionetteException("Instance PID '{0}' doesn't match the PID from capabilities {1}".format(self.handled_instance_process_id, self.process_id))
+            if self.instance_pid != self.process_id:
+                raise errors.MarionetteException("Instance PID '{0}' doesn't match the PID from capabilities {1}".format(self.instance_pid, self.process_id))
 
         self.profile = self.session.get("moz:profile")
 
         return self.session
 
     @property
     def test_name(self):
         return self._test_name