--- a/testing/marionette/client/marionette_driver/marionette.py
+++ b/testing/marionette/client/marionette_driver/marionette.py
@@ -569,16 +569,17 @@ class Marionette(object):
         """
         self.host = host
         self.port = self.local_port = int(port)
         self.bin = bin
         self.instance = None
         self.session = None
         self.session_id = None
         self.process_id = None
+        self.profile = None
         self.window = None
         self.chrome_window = None
         self.baseurl = baseurl
         self._test_name = None
         self.socket_timeout = socket_timeout
         self.crashed = 0
 
         startup_timeout = startup_timeout or self.DEFAULT_STARTUP_TIMEOUT
@@ -1269,16 +1270,17 @@ class Marionette(object):
 
         body = {"capabilities": desired_capabilities, "sessionId": session_id}
         resp = self._send_message("newSession", body)
 
         self.session_id = resp["sessionId"]
         self.session = resp["value"] if self.protocol == 1 else resp["capabilities"]
         # fallback to processId can be removed in Firefox 55
         self.process_id = self.session.get("moz:processID", self.session.get("processId"))
+        self.profile = self.session.get("moz:profile")
 
         return self.session
 
     @property
     def test_name(self):
         return self._test_name
 
     @test_name.setter
@@ -1299,16 +1301,17 @@ class Marionette(object):
         try:
             if send_request:
                 self._send_message("deleteSession")
         finally:
             if reset_session_id:
                 self.session_id = None
             self.session = None
             self.process_id = None
+            self.profile = None
             self.window = None
             self.client.close()
 
     @property
     def session_capabilities(self):
         """A JSON dictionary representing the capabilities of the
         current session.
 

--- a/testing/marionette/driver.js
+++ b/testing/marionette/driver.js
@@ -156,16 +156,17 @@ this.GeckoDriver = function(appName, ser
     // supported features
     "raisesAccessibilityExceptions": false,
     "rotatable": this.appName == "B2G",
     "proxy": {},
 
     // proprietary extensions
     "specificationLevel": 0,
     "moz:processID": Services.appinfo.processID,
+    "moz:profile": Services.dirsvc.get("ProfD", Ci.nsIFile).path,
   };
 
   this.mm = globalMessageManager;
   this.listener = proxy.toListener(() => this.mm, this.sendAsync.bind(this));
 
   // always keep weak reference to current dialogue
   this.dialog = null;
   let handleDialog = (subject, topic) => {

--- a/testing/marionette/harness/marionette/tests/unit/test_capabilities.py
+++ b/testing/marionette/harness/marionette/tests/unit/test_capabilities.py
@@ -37,16 +37,21 @@ class TestCapabilities(MarionetteTestCas
         self.assertIn("acceptInsecureCerts", self.caps)
         self.assertFalse(self.caps["acceptInsecureCerts"])
 
     def test_additional_capabilities(self):
         self.assertIn("moz:processID", self.caps)
         self.assertEqual(self.caps["moz:processID"], self.appinfo["processID"])
         self.assertEqual(self.marionette.process_id, self.appinfo["processID"])
 
+        current_profile = self.marionette.instance.runner.profile.profile
+        self.assertIn("moz:profile", self.caps)
+        self.assertEqual(self.caps["moz:profile"], current_profile)
+        self.assertEqual(self.marionette.profile, current_profile)
+
     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):