--- a/layout/tools/reftest/runreftest.py
+++ b/layout/tools/reftest/runreftest.py
@@ -68,17 +68,17 @@ class RefTest(object):
     if options.ignoreWindowSize != False:
       prefsFile.write('user_pref("reftest.ignoreWindowSize", true);\n')
     if options.filter != None:
       prefsFile.write('user_pref("reftest.filter", %s);\n' % self.makeJSString(options.filter))
     prefsFile.write('user_pref("reftest.focusFilterMode", %s);\n' % self.makeJSString(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
+    prefsFile.write('user_pref("toolkit.telemetry.enabled", false);\n')
 
     for v in options.extraPrefs:
       thispref = v.split("=")
       if len(thispref) < 2:
         print "Error: syntax error in --setpref=" + v
         sys.exit(1)
       part = 'user_pref("%s", %s);\n' % (thispref[0], thispref[1])
       prefsFile.write(part)