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diff --git a/devtools/startup/tests/xpcshell/test_devtools_shim.js b/devtools/startup/tests/xpcshell/test_devtools_shim.js
new file mode 100644
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+++ b/devtools/startup/tests/xpcshell/test_devtools_shim.js
@@ -0,0 +1,200 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+const { DevToolsShim } = ChromeUtils.importESModule(
+  "chrome://devtools-startup/content/DevToolsShim.sys.mjs"
+);
+
+// Test the DevToolsShim
+
+/**
+ * Create a mocked version of DevTools that records all calls made to methods expected
+ * to be called by DevToolsShim.
+ */
+function createMockDevTools() {
+  const methods = [
+    "on",
+    "off",
+    "emit",
+    "saveDevToolsSession",
+    "restoreDevToolsSession",
+  ];
+
+  const mock = {
+    callLog: {},
+  };
+
+  for (const method of methods) {
+    // Create a stub for method, that only pushes its arguments in the inner callLog
+    mock[method] = function (...args) {
+      mock.callLog[method].push(args);
+    };
+    mock.callLog[method] = [];
+  }
+
+  return mock;
+}
+
+/**
+ * Check if a given method was called an expected number of times, and finally check the
+ * arguments provided to the last call, if appropriate.
+ */
+function checkCalls(mock, method, length, lastArgs) {
+  ok(
+    mock.callLog[method].length === length,
+    "Devtools.on was called the expected number of times"
+  );
+
+  // If we don't want to check the last call or if the method was never called, bail out.
+  if (!lastArgs || length === 0) {
+    return;
+  }
+
+  for (let i = 0; i < lastArgs.length; i++) {
+    const expectedArg = lastArgs[i];
+    ok(
+      mock.callLog[method][length - 1][i] === expectedArg,
+      `Devtools.${method} was called with the expected argument (index ${i})`
+    );
+  }
+}
+
+function test_register_unregister() {
+  ok(!DevToolsShim.isInitialized(), "DevTools are not initialized");
+
+  DevToolsShim.register(createMockDevTools());
+  ok(DevToolsShim.isInitialized(), "DevTools are initialized");
+
+  DevToolsShim.unregister();
+  ok(!DevToolsShim.isInitialized(), "DevTools are not initialized");
+}
+
+function test_on_is_forwarded_to_devtools() {
+  ok(!DevToolsShim.isInitialized(), "DevTools are not initialized");
+
+  function cb1() {}
+  function cb2() {}
+  const mock = createMockDevTools();
+
+  DevToolsShim.on("test_event", cb1);
+  DevToolsShim.register(mock);
+  checkCalls(mock, "on", 1, ["test_event", cb1]);
+
+  DevToolsShim.on("other_event", cb2);
+  checkCalls(mock, "on", 2, ["other_event", cb2]);
+}
+
+function test_off_called_before_registering_devtools() {
+  ok(!DevToolsShim.isInitialized(), "DevTools are not initialized");
+
+  function cb1() {}
+  const mock = createMockDevTools();
+
+  DevToolsShim.on("test_event", cb1);
+  DevToolsShim.off("test_event", cb1);
+
+  DevToolsShim.register(mock);
+  checkCalls(mock, "on", 0);
+}
+
+function test_off_called_before_with_bad_callback() {
+  ok(!DevToolsShim.isInitialized(), "DevTools are not initialized");
+
+  function cb1() {}
+  function cb2() {}
+  const mock = createMockDevTools();
+
+  DevToolsShim.on("test_event", cb1);
+  DevToolsShim.off("test_event", cb2);
+
+  DevToolsShim.register(mock);
+  // on should still be called
+  checkCalls(mock, "on", 1, ["test_event", cb1]);
+  // Calls to off should not be held and forwarded.
+  checkCalls(mock, "off", 0);
+}
+
+function test_events() {
+  ok(!DevToolsShim.isInitialized(), "DevTools are not initialized");
+
+  const mock = createMockDevTools();
+  // Check emit was not called.
+  checkCalls(mock, "emit", 0);
+
+  // Check emit is called once with the devtools-registered event.
+  DevToolsShim.register(mock);
+  checkCalls(mock, "emit", 1, ["devtools-registered"]);
+
+  // Check emit is called once with the devtools-unregistered event.
+  DevToolsShim.unregister();
+  checkCalls(mock, "emit", 2, ["devtools-unregistered"]);
+}
+
+function test_restore_session_apis() {
+  // Backup method that will be updated for the test.
+  const initDevToolsBackup = DevToolsShim.initDevTools;
+
+  // Create fake session objects to restore.
+  const sessionWithoutDevTools = {};
+  const sessionWithDevTools = {
+    browserConsole: true,
+  };
+
+  Services.prefs.setBoolPref("devtools.policy.disabled", true);
+  ok(!DevToolsShim.isInitialized(), "DevTools are not initialized");
+  ok(!DevToolsShim.isEnabled(), "DevTools are not enabled");
+
+  // Check that save & restore DevToolsSession don't initialize the tools and don't
+  // crash.
+  DevToolsShim.saveDevToolsSession({});
+  DevToolsShim.restoreDevToolsSession(sessionWithDevTools);
+  ok(!DevToolsShim.isInitialized(), "DevTools are still not initialized");
+
+  Services.prefs.setBoolPref("devtools.policy.disabled", false);
+  ok(DevToolsShim.isEnabled(), "DevTools are enabled");
+  ok(!DevToolsShim.isInitialized(), "DevTools are not initialized");
+
+  // Check that DevTools are not initialized when calling restoreDevToolsSession without
+  // DevTools related data.
+  DevToolsShim.restoreDevToolsSession(sessionWithoutDevTools);
+  ok(!DevToolsShim.isInitialized(), "DevTools are still not initialized");
+
+  const mock = createMockDevTools();
+  DevToolsShim.initDevTools = () => {
+    // Next call to restoreDevToolsSession is expected to initialize DevTools, which we
+    // simulate here by registering our mock.
+    DevToolsShim.register(mock);
+  };
+
+  DevToolsShim.restoreDevToolsSession(sessionWithDevTools);
+  checkCalls(mock, "restoreDevToolsSession", 1, [sessionWithDevTools]);
+
+  ok(DevToolsShim.isInitialized(), "DevTools are initialized");
+
+  DevToolsShim.saveDevToolsSession({});
+  checkCalls(mock, "saveDevToolsSession", 1, []);
+
+  // Restore initDevTools backup.
+  DevToolsShim.initDevTools = initDevToolsBackup;
+}
+
+function run_test() {
+  test_register_unregister();
+  DevToolsShim.unregister();
+
+  test_on_is_forwarded_to_devtools();
+  DevToolsShim.unregister();
+
+  test_off_called_before_registering_devtools();
+  DevToolsShim.unregister();
+
+  test_off_called_before_with_bad_callback();
+  DevToolsShim.unregister();
+
+  test_restore_session_apis();
+  DevToolsShim.unregister();
+
+  test_events();
+}