--- a/modules/libpref/init/all.js
+++ b/modules/libpref/init/all.js
@@ -5538,24 +5538,25 @@ pref("media.block-autoplay-until-in-fore
 #ifdef MOZ_STYLO
 // Is the Servo-backed style system enabled?
 pref("layout.css.servo.enabled", true);
 #endif
 
 // HSTS Priming
 // If a request is mixed-content, send an HSTS priming request to attempt to
 // see if it is available over HTTPS.
-pref("security.mixed_content.send_hsts_priming", true);
 #ifdef RELEASE_OR_BETA
 // Don't change the order of evaluation of mixed-content and HSTS upgrades in
 // order to be most compatible with current standards
+pref("security.mixed_content.send_hsts_priming", false);
 pref("security.mixed_content.use_hsts", false);
 #else
 // Change the order of evaluation so HSTS upgrades happen before
 // mixed-content blocking
+pref("security.mixed_content.send_hsts_priming", true);
 pref("security.mixed_content.use_hsts", true);
 #endif
 // Approximately 1 week default cache for HSTS priming failures, in seconds
 pref ("security.mixed_content.hsts_priming_cache_timeout", 10080);
 // Force the channel to timeout in 3 seconds if we have not received
 // expects a time in milliseconds
 pref ("security.mixed_content.hsts_priming_request_timeout", 3000);
 

--- a/netwerk/base/security-prefs.js
+++ b/netwerk/base/security-prefs.js
@@ -102,19 +102,27 @@ pref("security.ssl.errorReporting.enable
 pref("security.ssl.errorReporting.url", "https://incoming.telemetry.mozilla.org/submit/sslreports/");
 pref("security.ssl.errorReporting.automatic", false);
 
 // Impose a maximum age on HPKP headers, to avoid sites getting permanently
 // blacking themselves out by setting a bad pin.  (60 days by default)
 // https://tools.ietf.org/html/rfc7469#section-4.1
 pref("security.cert_pinning.max_max_age_seconds", 5184000);
 
+// HSTS Priming
 // If a request is mixed-content, send an HSTS priming request to attempt to
 // see if it is available over HTTPS.
-pref("security.mixed_content.send_hsts_priming", true);
 #ifdef RELEASE_OR_BETA
-// Don't change the order of evaluation of mixed-content and HSTS upgrades
+// Don't change the order of evaluation of mixed-content and HSTS upgrades in
+// order to be most compatible with current standards
+pref("security.mixed_content.send_hsts_priming", false);
 pref("security.mixed_content.use_hsts", false);
 #else
 // Change the order of evaluation so HSTS upgrades happen before
 // mixed-content blocking
+pref("security.mixed_content.send_hsts_priming", true);
 pref("security.mixed_content.use_hsts", true);
 #endif
+// Approximately 1 week default cache for HSTS priming failures, in seconds
+pref ("security.mixed_content.hsts_priming_cache_timeout", 10080);
+// Force the channel to timeout in 3 seconds if we have not received
+// expects a time in milliseconds
+pref ("security.mixed_content.hsts_priming_request_timeout", 3000);