--- a/dom/ipc/moz.build
+++ b/dom/ipc/moz.build
@@ -168,16 +168,17 @@ if CONFIG['MOZ_WIDGET_TOOLKIT'] == 'andr
 if CONFIG['MOZ_TOOLKIT_SEARCH']:
     DEFINES['MOZ_TOOLKIT_SEARCH'] = True
 
 JAR_MANIFESTS += ['jar.mn']
 
 BROWSER_CHROME_MANIFESTS += ['tests/browser.ini']
 MOCHITEST_CHROME_MANIFESTS += ['tests/chrome.ini']
 MOCHITEST_MANIFESTS += ['tests/mochitest.ini']
+XPCSHELL_TESTS_MANIFESTS += ['tests/xpcshell.ini']
 
 CXXFLAGS += CONFIG['TK_CFLAGS']
 
 if CONFIG['CC_TYPE'] in ('clang', 'gcc'):
     CXXFLAGS += ['-Wno-error=shadow']
 
 if CONFIG['NIGHTLY_BUILD']:
     DEFINES['ASYNC_CONTENTPROC_LAUNCH'] = True

new file mode 100644
--- /dev/null
+++ b/dom/ipc/tests/test_sharedMap.js
@@ -0,0 +1,133 @@
+"use strict";
+
+ChromeUtils.import("resource://gre/modules/Services.jsm");
+ChromeUtils.import("resource://testing-common/ExtensionXPCShellUtils.jsm");
+
+ExtensionTestUtils.init(this);
+
+let contentPage;
+
+function getContents(sharedMap = Services.cpmm.sharedData) {
+  return {
+    keys: Array.from(sharedMap.keys()),
+    values: Array.from(sharedMap.values()),
+    entries: Array.from(sharedMap.entries()),
+    getValues: Array.from(sharedMap.keys(),
+                          key => sharedMap.get(key)),
+  };
+}
+
+function checkMap(contents, expected) {
+  expected = Array.from(expected);
+
+  equal(contents.keys.length, expected.length,
+        "Got correct number of keys");
+  equal(contents.values.length, expected.length,
+        "Got correct number of values");
+  equal(contents.entries.length, expected.length,
+        "Got correct number of entries");
+
+  for (let [i, [key, val]] of contents.entries.entries()) {
+    equal(key, contents.keys[i], `keys()[${i}] matches entries()[${i}]`);
+    deepEqual(val, contents.values[i], `values()[${i}] matches entries()[${i}]`);
+  }
+
+  expected.sort(([a], [b]) => a.localeCompare(b));
+  contents.entries.sort(([a], [b]) => a.localeCompare(b));
+
+  for (let [i, [key, val]] of contents.entries.entries()) {
+    equal(key, expected[i][0], `expected[${i}].key matches entries()[${i}].key`);
+    deepEqual(val, expected[i][1], `expected[${i}].value matches entries()[${i}].value`);
+  }
+}
+
+function checkParentMap(expected) {
+  info("Checking parent map");
+  checkMap(getContents(Services.ppmm.sharedData), expected);
+}
+
+async function checkContentMaps(expected) {
+  info("Checking in-process content map");
+  checkMap(getContents(Services.cpmm.sharedData), expected);
+
+  info("Checking out-of-process content map");
+  let contents = await contentPage.spawn(undefined, getContents);
+  checkMap(contents, expected);
+}
+
+add_task(async function setup() {
+  contentPage = await ExtensionTestUtils.loadContentPage("about:blank");
+  registerCleanupFunction(() => contentPage.close());
+});
+
+add_task(async function test_sharedMap() {
+  let {sharedData} = Services.ppmm;
+
+  info("Check that parent and child maps are both initially empty");
+
+  checkParentMap([]);
+  await checkContentMaps([]);
+
+  let expected = [
+    ["foo-a", {"foo": "a"}],
+    ["foo-b", {"foo": "b"}],
+    ["bar-c", null],
+    ["bar-d", 42],
+  ];
+
+  function setKey(key, val) {
+    sharedData.set(key, val);
+    expected = expected.filter(([k]) => k != key);
+    expected.push([key, val]);
+  }
+  function deleteKey(key) {
+    sharedData.delete(key);
+    expected = expected.filter(([k]) => k != key);
+  }
+
+  for (let [key, val] of expected) {
+    sharedData.set(key, val);
+  }
+
+  info("Add some entries, test that they are initially only available in the parent");
+
+  checkParentMap(expected);
+  await checkContentMaps([]);
+
+  info("Flush. Check that changes are visible in both parent and children");
+
+  sharedData.flush();
+
+  checkParentMap(expected);
+  await checkContentMaps(expected);
+
+  info("Add another entry. Check that it is initially only available in the parent");
+
+  let oldExpected = Array.from(expected);
+
+  setKey("baz-a", {meh: "meh"});
+
+  checkParentMap(expected);
+  await checkContentMaps(oldExpected);
+
+  info("Add another entry. Check that both new entries are only available in the parent");
+
+  setKey("baz-a", {meh: 12});
+
+  checkParentMap(expected);
+  await checkContentMaps(oldExpected);
+
+  info("Delete an entry. Check that all changes are only visible in the parent");
+
+  deleteKey("foo-b");
+
+  checkParentMap(expected);
+  await checkContentMaps(oldExpected);
+
+  info("Flush. Check that all entries are available in both parent and children");
+
+  sharedData.flush();
+
+  checkParentMap(expected);
+  await checkContentMaps(expected);
+});

new file mode 100644
--- /dev/null
+++ b/dom/ipc/tests/xpcshell.ini
@@ -0,0 +1,2 @@
+
+[test_sharedMap.js]