--- a/layout/tools/reftest/reftest-preferences.js
+++ b/layout/tools/reftest/reftest-preferences.js
@@ -41,10 +41,8 @@
     // Disable interruptible reflow since (1) it's normally not going to
     // happen, but (2) it might happen if we somehow end up with both
     // pending user events and clock skew.  So to avoid having to change
     // MakeProgress to deal with waiting for interruptible reflows to
     // complete for a rare edge case, we just disable interruptible
     // reflow so that that rare edge case doesn't lead to reftest
     // failures.
     branch.setBoolPref("layout.interruptible-reflow.enabled", false);
-    // Don't try to connect to the telemetry server.
-    branch.setBoolPref("toolkit.telemetry.enabled", false);

--- a/layout/tools/reftest/runreftest.py
+++ b/layout/tools/reftest/runreftest.py
@@ -160,16 +160,20 @@ class RefTest(object):
     if options.ignoreWindowSize:
       prefs['reftest.ignoreWindowSize'] = True
     if options.filter:
       prefs['reftest.filter'] = options.filter
     if options.shuffle:
       prefs['reftest.shuffle'] = True
     prefs['reftest.focusFilterMode'] = options.focusFilterMode
 
+    # Ensure that telemetry is disabled, so we don't connect to the telemetry
+    # server in the middle of the tests.
+    prefs['toolkit.telemetry.enabled'] = False
+
     if options.e10s:
       prefs['browser.tabs.remote.autostart'] = True
 
     for v in options.extraPrefs:
       thispref = v.split('=')
       if len(thispref) < 2:
         print "Error: syntax error in --setpref=" + v
         sys.exit(1)