--- a/testing/mozharness/mozharness/mozilla/firefox/autoconfig.py
+++ b/testing/mozharness/mozharness/mozilla/firefox/autoconfig.py
@@ -26,16 +26,25 @@ def write_autoconfig_files(fx_install_di
     autoconfig_contents - autoconfig.js content to refer to .cfg gile
     '''
     with open(_cfg_file_path(fx_install_dir), 'w') as fd:
         fd.write(cfg_contents)
     with open(_autoconfig_path(fx_install_dir), 'w') as fd:
         fd.write(autoconfig_contents)
 
 
+def read_autoconfig_file(fx_install_dir):
+    ''' Read autoconfig file that modifies Firefox startup
+
+    fx_install_dir - path to Firefox installation
+    '''
+    with open(_cfg_file_path(fx_install_dir), 'r') as fd:
+        return fd.read()
+
+
 def _autoconfig_path(fx_install_dir):
     platform = platform_name()
     if platform in ('win32', 'win64'):
         return os.path.join(fx_install_dir,
                             'defaults', 'pref', 'autoconfig.js')
     elif platform in ('linux', 'linux64'):
         return os.path.join(fx_install_dir,
                             'defaults/pref/autoconfig.js')

--- a/testing/talos/talos/mitmproxy/mitmproxy.py
+++ b/testing/talos/talos/mitmproxy/mitmproxy.py
@@ -66,35 +66,48 @@ def _read_certificate(certificate_path):
     ''' Return the certificate's hash from the certificate file.'''
     # NOTE: mitmproxy's certificates do not exist until one of its binaries
     #       has been executed once on the host
     with open(certificate_path, 'r') as fd:
         contents = fd.read()
     return ''.join(contents.splitlines()[1:-1])
 
 
-def is_mitmproxy_cert_installed():
+def is_mitmproxy_cert_installed(browser_install):
     """Verify mitmxproy CA cert was added to Firefox"""
-    # TODO: Bug 1366071
+    from mozharness.mozilla.firefox.autoconfig import read_autoconfig_file
+    try:
+        # read autoconfig file, confirm mitmproxy cert is in there
+        certificate = _read_certificate(DEFAULT_CERT_PATH)
+        contents = read_autoconfig_file(browser_install)
+        if (MITMPROXY_SETTINGS % {'cert': certificate}) in contents:
+            LOG.info("Verified mitmproxy CA certificate is installed in Firefox")
+        else:
+            LOG.info("Firefox autoconfig file contents:")
+            LOG.info(contents)
+            return False
+    except:
+        LOG.info("Failed to read Firefox autoconfig file, when verifying CA certificate install")
+        return False
     return True
 
 
 def install_mitmproxy_cert(mitmproxy_proc, browser_path, scripts_path):
     """Install the CA certificate generated by mitmproxy, into Firefox"""
     LOG.info("Installing mitmxproxy CA certficate into Firefox")
     # browser_path is exe, we want install dir
     browser_install = os.path.dirname(browser_path)
     # on macosx we need to remove the last folders 'Content/MacOS'
     if mozinfo.os == 'mac':
         browser_install = browser_install[:-14]
 
     LOG.info('Calling configure_mitmproxy with browser folder: %s' % browser_install)
     configure_mitmproxy(browser_install, scripts_path)
     # cannot continue if failed to add CA cert to Firefox, need to check
-    if not is_mitmproxy_cert_installed():
+    if not is_mitmproxy_cert_installed(browser_install):
         LOG.error('Aborting: failed to install mitmproxy CA cert into Firefox')
         stop_mitmproxy_playback(mitmproxy_proc)
         sys.exit()
 
 
 def start_mitmproxy_playback(mitmdump_path,
                              mitmproxy_recording_path,
                              mitmproxy_recordings_list,