--- a/toolkit/components/telemetry/tests/marionette/harness/telemetry_harness/runner.py
+++ b/toolkit/components/telemetry/tests/marionette/harness/telemetry_harness/runner.py
@@ -24,18 +24,16 @@ class TelemetryTestRunner(BaseMarionette
 
         # Set Firefox Client Telemetry specific preferences
         prefs.update(
             {
                 # Fake the geoip lookup to always return Germany to:
                 #   * avoid net access in tests
                 #   * stabilize browser.search.region to avoid an extra subsession (bug 1545207)
                 "browser.search.geoip.url": "data:application/json,{\"country_code\": \"DE\"}",
-                # Disable smart sizing because it changes prefs at startup. (bug 1547750)
-                "browser.cache.disk.smart_size.enabled": False,
                 "toolkit.telemetry.server": "{}/pings".format(SERVER_URL),
                 "toolkit.telemetry.initDelay": 1,
                 "toolkit.telemetry.minSubsessionLength": 0,
                 "datareporting.healthreport.uploadEnabled": True,
                 "datareporting.policy.dataSubmissionEnabled": True,
                 "datareporting.policy.dataSubmissionPolicyBypassNotification": True,
                 "toolkit.telemetry.log.level": "Trace",
                 "toolkit.telemetry.log.dump": True,