--- a/testing/marionette/client/marionette_driver/marionette.py
+++ b/testing/marionette/client/marionette_driver/marionette.py
@@ -675,17 +675,17 @@ class Marionette(object):
                     data["parameters"] = params
                 self.client.send(data)
                 msg = self.client.receive()
 
             else:
                 msg = self.client.request(name, params)
 
         except IOError:
-            if self.instance and not hasattr(self.instance, 'detached'):
+            if self.instance:
                 # If we've launched the binary we've connected to, wait
                 # for it to shut down.
                 returncode = self.instance.runner.wait(timeout=self.DEFAULT_STARTUP_TIMEOUT)
                 raise IOError("process died with returncode %s" % returncode)
             raise
 
         except socket.timeout:
             self.session = None
@@ -1033,26 +1033,30 @@ class Marionette(object):
             # Values here correspond to constants in nsIAppStartup.
             # See http://mzl.la/1X0JZsC
             restart_flags = [
                 "eForceQuit",
                 "eRestart",
             ]
             self._send_message("quitApplication", {"flags": restart_flags})
             self.client.close()
-            # The instance is restarting itself; we will no longer be able to
-            # track it by pid, so mark it as 'detached'.
-            self.instance.detached = True
         else:
             self.delete_session()
             self.instance.restart(clean=clean)
+
         self.raise_for_port(self.wait_for_port())
         self.start_session(session_id=self.session_id)
         self._reset_timeouts()
 
+        if in_app:
+            # In some cases Firefox restarts itself by spawning into a new process group.
+            # As long as mozprocess cannot track that behavior (bug 1284864) we assist by
+            # informing about the new process id.
+            self.instance.runner.process_handler.check_for_detached(self.session['processId'])
+
     def absolute_url(self, relative_url):
         '''
         Returns an absolute url for files served from Marionette's www directory.
 
         :param relative_url: The url of a static file, relative to Marionette's www directory.
         '''
         return "%s%s" % (self.baseurl, relative_url)
 

--- a/testing/marionette/client/requirements.txt
+++ b/testing/marionette/client/requirements.txt
@@ -1,1 +1,1 @@
-mozrunner >= 6.9
+mozrunner >= 6.12