--- a/testing/marionette/driver.js
+++ b/testing/marionette/driver.js
@@ -146,17 +146,16 @@ this.GeckoDriver = function(appName, ser
     "takesElementScreenshot": true,
     "takesScreenshot": true,
     "proxy": {},
 
     // Selenium 2 compat
     "platform": Services.sysinfo.getProperty("name").toUpperCase(),
 
     // proprietary extensions
-    "XULappId" : Services.appinfo.ID,
     "appBuildId" : Services.appinfo.appBuildID,
     "processId" : Services.appinfo.processID,
   };
 
   this.mm = globalMessageManager;
   this.listener = proxy.toListener(() => this.mm, this.sendAsync.bind(this));
 
   // always keep weak reference to current dialogue

--- a/testing/marionette/harness/marionette/tests/unit/test_capabilities.py
+++ b/testing/marionette/harness/marionette/tests/unit/test_capabilities.py
@@ -42,20 +42,18 @@ class TestCapabilities(MarionetteTestCas
         self.assertTrue(self.caps["takesElementScreenshot"])
         self.assertTrue(self.caps["takesScreenshot"])
 
     def test_selenium2_compat(self):
         self.assertIn("platform", self.caps)
         self.assertEqual(self.caps["platform"], self.caps["platformName"].upper())
 
     def test_extensions(self):
-        self.assertIn("XULappId", self.caps)
         self.assertIn("appBuildId", self.caps)
 
-        self.assertEqual(self.caps["XULappId"], self.appinfo["ID"])
         self.assertEqual(self.caps["appBuildId"], self.appinfo["appBuildID"])
 
     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)