diff --git a/accessible/moz.build b/accessible/moz.build
index 6d7abf93b5c9..67373fd3a8ca 100644
--- a/accessible/moz.build
+++ b/accessible/moz.build
@@ -48,6 +48,7 @@ BROWSER_CHROME_MANIFESTS += [
     "tests/browser/telemetry/browser.toml",
     "tests/browser/text/browser.toml",
     "tests/browser/tree/browser.toml",
+    "tests/browser/windows/ia2/browser.toml",
 ]
 
 with Files("**"):
diff --git a/accessible/tests/browser/browser.toml b/accessible/tests/browser/browser.toml
index 45c2d3059f95..e2ae7407f9a7 100644
--- a/accessible/tests/browser/browser.toml
+++ b/accessible/tests/browser/browser.toml
@@ -7,6 +7,8 @@ support-files = [
   "head.js",
   "python_runner_wsh.py",
   "shared-head.js",
+  "windows/a11y_setup.py",
+  "windows/a11y_setup_requirements.txt",
 ]
 prefs = ["javascript.options.asyncstack_capture_debuggee_only=false"]
 
diff --git a/accessible/tests/browser/python_runner_wsh.py b/accessible/tests/browser/python_runner_wsh.py
index 6581174325ee..d10bc4afe292 100644
--- a/accessible/tests/browser/python_runner_wsh.py
+++ b/accessible/tests/browser/python_runner_wsh.py
@@ -9,6 +9,8 @@ It is intended to be called from JS browser tests.
 """
 
 import json
+import os
+import sys
 import traceback
 
 from mod_pywebsocket import msgutil
@@ -23,9 +25,27 @@ def web_socket_transfer_data(request):
         """Send a response to the client as a JSON array."""
         msgutil.send_message(request, json.dumps(args))
 
-    # XXX Anything that should be accessible to code run
-    # via the WebSocket should be added to cleanNamespace.
     cleanNamespace = {}
+    testDir = None
+    if sys.platform == "win32":
+        testDir = "windows"
+    elif sys.platform == "linux":
+        # XXX ATK code goes here.
+        pass
+    if testDir:
+        sys.path.append(
+            os.path.join(
+                os.getcwd(), "browser", "accessible", "tests", "browser", testDir
+            )
+        )
+        try:
+            import a11y_setup
+
+            cleanNamespace = a11y_setup.__dict__
+            setupExc = None
+        except Exception:
+            setupExc = traceback.format_exc()
+        sys.path.pop()
 
     def info(message):
         """Log an info message."""
@@ -43,6 +63,11 @@ def web_socket_transfer_data(request):
             namespace = cleanNamespace.copy()
             continue
 
+        if setupExc:
+            # a11y_setup failed. Report an exception immediately.
+            send("exception", setupExc)
+            continue
+
         # Wrap the code in a function called run(). This allows the code to
         # return a result by simply using the return statement.
         if "\n" not in code and not code.lstrip().startswith("return "):
diff --git a/accessible/tests/browser/shared-head.js b/accessible/tests/browser/shared-head.js
index 6f65093f2c6a..fe87a777650f 100644
--- a/accessible/tests/browser/shared-head.js
+++ b/accessible/tests/browser/shared-head.js
@@ -935,6 +935,8 @@ let gPythonSocket = null;
  * be resolved with the deserialized result. If the Python code raises an
  * exception, the JS Promise will be rejected with the Python traceback.
  * An info() function is provided in Python to log an info message.
+ * See windows/a11y_setup.py for other things available in the Python
+ * environment.
  */
 function runPython(code) {
   if (!gPythonSocket) {
diff --git a/accessible/tests/browser/windows/a11y_setup.py b/accessible/tests/browser/windows/a11y_setup.py
new file mode 100644
index 000000000000..e7bdeda35ba5
--- /dev/null
+++ b/accessible/tests/browser/windows/a11y_setup.py
@@ -0,0 +1,116 @@
+# This Source Code Form is subject to the terms of the Mozilla Public
+# License, v. 2.0. If a copy of the MPL was not distributed with this
+# file, You can obtain one at http://mozilla.org/MPL/2.0/.
+
+"""Python environment for Windows a11y browser tests.
+"""
+
+import ctypes
+import os
+from ctypes import POINTER, byref
+from ctypes.wintypes import BOOL, HWND, LPARAM
+
+import comtypes.client
+import psutil
+from comtypes import COMError, IServiceProvider
+
+user32 = ctypes.windll.user32
+oleacc = ctypes.oledll.oleacc
+oleaccMod = comtypes.client.GetModule("oleacc.dll")
+IAccessible = oleaccMod.IAccessible
+del oleaccMod
+OBJID_CLIENT = -4
+CHILDID_SELF = 0
+NAVRELATION_EMBEDS = 0x1009
+# This is the path if running locally.
+ia2Tlb = os.path.join(
+    os.getcwd(),
+    "..",
+    "..",
+    "..",
+    "accessible",
+    "interfaces",
+    "ia2",
+    "IA2Typelib.tlb",
+)
+if not os.path.isfile(ia2Tlb):
+    # This is the path if running in CI.
+    ia2Tlb = os.path.join(os.getcwd(), "ia2Typelib.tlb")
+ia2Mod = comtypes.client.GetModule(ia2Tlb)
+del ia2Tlb
+# Shove all the IAccessible* interfaces and IA2_* constants directly
+# into our namespace for convenience.
+globals().update((k, getattr(ia2Mod, k)) for k in ia2Mod.__all__)
+# We use this below. The linter doesn't understand our globals() update hack.
+IAccessible2 = ia2Mod.IAccessible2
+del ia2Mod
+
+
+def AccessibleObjectFromWindow(hwnd, objectID=OBJID_CLIENT):
+    p = POINTER(IAccessible)()
+    oleacc.AccessibleObjectFromWindow(
+        hwnd, objectID, byref(IAccessible._iid_), byref(p)
+    )
+    return p
+
+
+def getFirefoxHwnd():
+    """Search all top level windows for the Firefox instance being
+    tested.
+    We search by window class name and window title prefix.
+    """
+    # We can compare the grandparent process ids to find the Firefox started by
+    # the test harness.
+    commonPid = psutil.Process().parent().ppid()
+    # We need something mutable to store the result from the callback.
+    found = []
+
+    @ctypes.WINFUNCTYPE(BOOL, HWND, LPARAM)
+    def callback(hwnd, lParam):
+        name = ctypes.create_unicode_buffer(100)
+        user32.GetClassNameW(hwnd, name, 100)
+        if name.value != "MozillaWindowClass":
+            return True
+        pid = ctypes.wintypes.DWORD()
+        user32.GetWindowThreadProcessId(hwnd, byref(pid))
+        if psutil.Process(pid.value).parent().ppid() != commonPid:
+            return True  # Not the Firefox being tested.
+        found.append(hwnd)
+        return False
+
+    user32.EnumWindows(callback, LPARAM(0))
+    if not found:
+        raise LookupError("Couldn't find Firefox HWND")
+    return found[0]
+
+
+def toIa2(obj):
+    serv = obj.QueryInterface(IServiceProvider)
+    return serv.QueryService(IAccessible2._iid_, IAccessible2)
+
+
+def getDocIa2():
+    """Get the IAccessible2 for the document being tested."""
+    hwnd = getFirefoxHwnd()
+    root = AccessibleObjectFromWindow(hwnd)
+    doc = root.accNavigate(NAVRELATION_EMBEDS, 0)
+    try:
+        child = toIa2(doc.accChild(1))
+        if "id:default-iframe-id;" in child.attributes:
+            # This is an iframe or remoteIframe test.
+            doc = child.accChild(1)
+    except COMError:
+        pass  # No child.
+    return toIa2(doc)
+
+
+def findIa2ByDomId(root, id):
+    search = f"id:{id};"
+    # Child ids begin at 1.
+    for i in range(1, root.accChildCount + 1):
+        child = toIa2(root.accChild(i))
+        if search in child.attributes:
+            return child
+        descendant = findIa2ByDomId(child, id)
+        if descendant:
+            return descendant
diff --git a/accessible/tests/browser/windows/a11y_setup_requirements.txt b/accessible/tests/browser/windows/a11y_setup_requirements.txt
new file mode 100644
index 000000000000..ea131a6f92f2
--- /dev/null
+++ b/accessible/tests/browser/windows/a11y_setup_requirements.txt
@@ -0,0 +1 @@
+comtypes==1.2.0
diff --git a/accessible/tests/browser/windows/ia2/browser.toml b/accessible/tests/browser/windows/ia2/browser.toml
new file mode 100644
index 000000000000..d72b5f8a2db2
--- /dev/null
+++ b/accessible/tests/browser/windows/ia2/browser.toml
@@ -0,0 +1,9 @@
+[DEFAULT]
+subsuite = "a11y"
+skip-if = [
+  "os != 'win'",
+  "headless",
+]
+support-files = ["head.js"]
+
+["browser_role.js"]
diff --git a/accessible/tests/browser/windows/ia2/browser_role.js b/accessible/tests/browser/windows/ia2/browser_role.js
new file mode 100644
index 000000000000..08e44c280f5c
--- /dev/null
+++ b/accessible/tests/browser/windows/ia2/browser_role.js
@@ -0,0 +1,39 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+"use strict";
+
+const ROLE_SYSTEM_DOCUMENT = 15;
+const ROLE_SYSTEM_GROUPING = 20;
+const IA2_ROLE_PARAGRAPH = 1054;
+
+addAccessibleTask(
+  `
+<p id="p">p</p>
+  `,
+  async function (browser, docAcc) {
+    let role = await runPython(`
+      global doc
+      doc = getDocIa2()
+      return doc.accRole(CHILDID_SELF)
+    `);
+    is(role, ROLE_SYSTEM_DOCUMENT, "doc has correct MSAA role");
+    role = await runPython(`doc.role()`);
+    is(role, ROLE_SYSTEM_DOCUMENT, "doc has correct IA2 role");
+    ok(
+      await runPython(`
+        global p
+        p = findIa2ByDomId(doc, "p")
+        firstChild = toIa2(doc.accChild(1))
+        return p == firstChild
+      `),
+      "doc's first child is p"
+    );
+    role = await runPython(`p.accRole(CHILDID_SELF)`);
+    is(role, ROLE_SYSTEM_GROUPING, "p has correct MSAA role");
+    role = await runPython(`p.role()`);
+    is(role, IA2_ROLE_PARAGRAPH, "p has correct IA2 role");
+  },
+  { chrome: true, topLevel: true, iframe: true, remoteIframe: true }
+);
diff --git a/accessible/tests/browser/windows/ia2/head.js b/accessible/tests/browser/windows/ia2/head.js
new file mode 100644
index 000000000000..afc50984bda3
--- /dev/null
+++ b/accessible/tests/browser/windows/ia2/head.js
@@ -0,0 +1,18 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+"use strict";
+
+// Load the shared-head file first.
+Services.scriptloader.loadSubScript(
+  "chrome://mochitests/content/browser/accessible/tests/browser/shared-head.js",
+  this
+);
+
+// Loading and common.js from accessible/tests/mochitest/ for all tests, as
+// well as promisified-events.js.
+loadScripts(
+  { name: "common.js", dir: MOCHITESTS_DIR },
+  { name: "promisified-events.js", dir: MOCHITESTS_DIR }
+);
diff --git a/python/mozbuild/mozbuild/action/test_archive.py b/python/mozbuild/mozbuild/action/test_archive.py
index 36ebf7aced67..af761ea662d1 100644
--- a/python/mozbuild/mozbuild/action/test_archive.py
+++ b/python/mozbuild/mozbuild/action/test_archive.py
@@ -287,6 +287,13 @@ ARCHIVE_FILES = {
             "pattern": "specialpowers/**",
             "dest": "mochitest/extensions",
         },
+        # Needed by Windows a11y browser tests.
+        {
+            "source": buildconfig.topobjdir,
+            "base": "accessible/interfaces/ia2",
+            "pattern": "IA2Typelib.tlb",
+            "dest": "mochitest",
+        },
     ],
     "mozharness": [
         {
diff --git a/testing/mochitest/mach_commands.py b/testing/mochitest/mach_commands.py
index 4a3427d19692..f719da42061f 100644
--- a/testing/mochitest/mach_commands.py
+++ b/testing/mochitest/mach_commands.py
@@ -403,6 +403,20 @@ def run_mochitest_general(
         # reason it doesn't get set when calling `activate_this.py` in the virtualenv.
         sys.executable = command_context.virtualenv_manager.python_path
 
+    if ("browser-chrome", "a11y") in suites and sys.platform == "win32":
+        # Only Windows a11y browser tests need this.
+        req = os.path.join(
+            "accessible",
+            "tests",
+            "browser",
+            "windows",
+            "a11y_setup_requirements.txt",
+        )
+        command_context.virtualenv_manager.activate()
+        command_context.virtualenv_manager.install_pip_requirements(
+            req, require_hashes=False
+        )
+
     # This is a hack to introduce an option in mach to not send
     # filtered tests to the mochitest harness. Mochitest harness will read
     # the master manifest in that case.
diff --git a/testing/mozharness/scripts/desktop_unittest.py b/testing/mozharness/scripts/desktop_unittest.py
index 05709d40e498..1aaa0803d862 100755
--- a/testing/mozharness/scripts/desktop_unittest.py
+++ b/testing/mozharness/scripts/desktop_unittest.py
@@ -527,6 +527,24 @@ class DesktopUnittest(TestingMixin, MercurialScript, MozbaseMixin, CodeCoverageM
                 )
             )
 
+        if (
+            self._query_specified_suites("mochitest", "mochitest-browser-a11y")
+            is not None
+            and sys.platform == "win32"
+        ):
+            # Only Windows a11y browser tests need this.
+            requirements_files.append(
+                os.path.join(
+                    dirs["abs_mochitest_dir"],
+                    "browser",
+                    "accessible",
+                    "tests",
+                    "browser",
+                    "windows",
+                    "a11y_setup_requirements.txt",
+                )
+            )
+
         for requirements_file in requirements_files:
             self.register_virtualenv_module(
                 requirements=[requirements_file], two_pass=True