--- a/layout/tools/reftest/reftest-preferences.js
+++ b/layout/tools/reftest/reftest-preferences.js
@@ -17,16 +17,18 @@ user_pref("hangmonitor.timeout", 0);
 // Ensure autoplay is enabled for all platforms.
 user_pref("media.autoplay.enabled", true);
 // Disable updates
 user_pref("app.update.enabled", false);
 user_pref("app.update.staging.enabled", false);
 user_pref("app.update.url.android", "");
 // Ensure we can load the reftest extension
 user_pref("extensions.allow-non-mpc-extensions", true);
+user_pref("extensions.legacy.enabled", true);
+user_pref("security.turn_off_all_security_so_that_viruses_can_take_over_this_computer", true);
 // Disable addon updates and prefetching so we don't leak them
 user_pref("extensions.update.enabled", false);
 user_pref("extensions.systemAddon.update.url", "http://localhost/dummy-system-addons.xml");
 user_pref("extensions.getAddons.cache.enabled", false);
 // Disable blocklist updates so we don't have them reported as leaks
 user_pref("extensions.blocklist.enabled", false);
 // Make url-classifier updates so rare that they won't affect tests
 user_pref("urlclassifier.updateinterval", 172800);

--- a/layout/tools/reftest/runreftest.py
+++ b/layout/tools/reftest/runreftest.py
@@ -320,20 +320,16 @@ class RefTest(object):
 
         addons = []
         if not self.use_marionette:
             addons.append(options.reftestExtensionPath)
 
         if options.specialPowersExtensionPath is not None:
             if not self.use_marionette:
                 addons.append(options.specialPowersExtensionPath)
-            # SpecialPowers requires insecure automation-only features that we
-            # put behind a pref.
-            prefs['security.turn_off_all_security_so_that_viruses'
-                  '_can_take_over_this_computer'] = True
 
         for pref in prefs:
             prefs[pref] = mozprofile.Preferences.cast(prefs[pref])
 
         # Install distributed extensions, if application has any.
         distExtDir = os.path.join(options.app[:options.app.rfind(os.sep)],
                                   "distribution", "extensions")
         if os.path.isdir(distExtDir):