--- a/testing/marionette/cookie.js
+++ b/testing/marionette/cookie.js
@@ -104,16 +104,19 @@ cookie.add = function(newCookie, {restri
   }
 
   let hostOnly = false;
   if (typeof newCookie.domain == "undefined") {
     hostOnly = true;
     newCookie.domain = restrictToHost;
   }
   assert.string(newCookie.domain, "Cookie domain must be string");
+  if (newCookie.domain.substring(0, 1) === ".") {
+    newCookie.domain = newCookie.domain.substring(1);
+  }
 
   if (typeof newCookie.secure == "undefined") {
     newCookie.secure = false;
   }
   if (typeof newCookie.httpOnly == "undefined") {
     newCookie.httpOnly = false;
   }
   if (typeof newCookie.expiry == "undefined") {

--- a/testing/marionette/test_cookie.js
+++ b/testing/marionette/test_cookie.js
@@ -207,16 +207,23 @@ add_test(function test_add() {
   cookie.add({
     name: "name5",
     value: "value5",
     domain: "domain5",
     path: "/foo/bar",
   });
   equal("/foo/bar", cookie.manager.cookies[3].path);
 
+  cookie.add({
+    name: "name6",
+    value: "value",
+    domain: ".domain"
+  });
+  equal(".domain", cookie.manager.cookies[4].host);
+
   Assert.throws(() => {
     cookie.add({name: "fail", value: "value6", domain: "domain6"})
   }, /UnableToSetCookieError/);
 
   run_next_test();
 });
 
 add_test(function test_remove() {

--- a/testing/web-platform/tests/webdriver/tests/cookies/add_cookie.py
+++ b/testing/web-platform/tests/webdriver/tests/cookies/add_cookie.py
@@ -1,20 +1,19 @@
 from tests.support.fixtures import clear_all_cookies
-from tests.support.fixtures import server_config
 from datetime import datetime, timedelta
 
-def test_add_domain_cookie(session, url):
+def test_add_domain_cookie(session, url, server_config):
     session.url = url("/common/blank.html")
     clear_all_cookies(session)
     create_cookie_request = {
         "cookie": {
             "name": "hello",
             "value": "world",
-            "domain": "web-platform.test",
+            "domain": server_config["domains"][""],
             "path": "/",
             "httpOnly": False,
             "secure": False
         }
     }
     result = session.transport.send("POST", "session/%s/cookie" % session.session_id, create_cookie_request)
     assert result.status == 200
     assert "value" in result.body
@@ -32,26 +31,26 @@ def test_add_domain_cookie(session, url)
     assert isinstance(cookie["name"], basestring)
     assert "value" in cookie
     assert isinstance(cookie["value"], basestring)
     assert "domain" in cookie
     assert isinstance(cookie["domain"], basestring)
 
     assert cookie["name"] == "hello"
     assert cookie["value"] == "world"
-    assert cookie["domain"] == ".web-platform.test"
+    assert cookie["domain"] == ".%s" % server_config["domains"][""]
 
-def test_add_cookie_for_ip(session, url, server_config):
+def test_add_cookie_for_ip(session, url, server_config, configuration):
     session.url = "http://127.0.0.1:%s/404" % (server_config["ports"]["http"][0])
     clear_all_cookies(session)
     create_cookie_request = {
         "cookie": {
             "name": "hello",
             "value": "world",
-            "domain": "127.0.0.1",
+            "domain": configuration["host"],
             "path": "/",
             "httpOnly": False,
             "secure": False
         }
     }
     result = session.transport.send("POST", "session/%s/cookie" % session.session_id, create_cookie_request)
     assert result.status == 200
     assert "value" in result.body
@@ -137,8 +136,42 @@ def test_add_session_cookie(session, url
     assert isinstance(cookie["name"], basestring)
     assert "value" in cookie
     assert isinstance(cookie["value"], basestring)
     assert "expiry" in cookie
     assert cookie.get("expiry") is None
 
     assert cookie["name"] == "hello"
     assert cookie["value"] == "world"
+
+def test_add_session_cookie_with_leading_dot_character_in_domain(session, url, server_config):
+    session.url = url("/common/blank.html")
+    clear_all_cookies(session)
+    create_cookie_request = {
+        "cookie": {
+            "name": "hello",
+            "value": "world",
+            "domain": ".%s" % server_config["domains"][""]
+        }
+    }
+    result = session.transport.send("POST", "session/%s/cookie" % session.session_id, create_cookie_request)
+    assert result.status == 200
+    assert "value" in result.body
+    assert isinstance(result.body["value"], dict)
+
+    result = session.transport.send("GET", "session/%s/cookie" % session.session_id)
+    assert result.status == 200
+    assert "value" in result.body
+    assert isinstance(result.body["value"], list)
+    assert len(result.body["value"]) == 1
+    assert isinstance(result.body["value"][0], dict)
+
+    cookie = result.body["value"][0]
+    assert "name" in cookie
+    assert isinstance(cookie["name"], basestring)
+    assert "value" in cookie
+    assert isinstance(cookie["value"], basestring)
+    assert "domain" in cookie
+    assert isinstance(cookie["domain"], basestring)
+
+    assert cookie["name"] == "hello"
+    assert cookie["value"] == "world"
+    assert cookie["domain"] == ".%s" % server_config["domains"][""]

--- a/testing/web-platform/tests/webdriver/tests/cookies/get_named_cookie.py
+++ b/testing/web-platform/tests/webdriver/tests/cookies/get_named_cookie.py
@@ -56,35 +56,35 @@ def test_get_named_cookie(session, url):
     assert "expiry" in cookie
     assert isinstance(cookie["expiry"], (int, long))
 
     assert cookie["name"] == "foo"
     assert cookie["value"] == "bar"
     # convert from seconds since epoch
     assert datetime.utcfromtimestamp(cookie["expiry"]).strftime(utc_string_format) == a_year_from_now
 
-def test_duplicated_cookie(session, url):
+def test_duplicated_cookie(session, url, server_config):
     session.url = url("/common/blank.html")
     clear_all_cookies(session)
     create_cookie_request = {
         "cookie": {
             "name": "hello",
             "value": "world",
-            "domain": "web-platform.test",
+            "domain": server_config["domains"][""],
             "path": "/",
             "httpOnly": False,
             "secure": False
         }
     }
     result = session.transport.send("POST", "session/%s/cookie" % session.session_id, create_cookie_request)
     assert result.status == 200
     assert "value" in result.body
     assert isinstance(result.body["value"], dict)
 
-    session.url = inline("<script>document.cookie = 'hello=newworld; domain=web-platform.test; path=/';</script>")
+    session.url = inline("<script>document.cookie = 'hello=newworld; domain=%s; path=/';</script>" % server_config["domains"][""])
     result = session.transport.send("GET", "session/%s/cookie" % session.session_id)
     assert result.status == 200
     assert "value" in result.body
     assert isinstance(result.body["value"], list)
     assert len(result.body["value"]) == 1
     assert isinstance(result.body["value"][0], dict)
 
     cookie = result.body["value"][0]