diff --git a/netwerk/base/security-prefs.js b/netwerk/base/security-prefs.js
index f22a49444b..d0adccd879 100644
--- a/netwerk/base/security-prefs.js
+++ b/netwerk/base/security-prefs.js
@@ -118,6 +118,15 @@ pref("security.ssl.enable_ocsp_must_staple", true);
 // Restart required.
 pref("security.ssl.enable_tls13_compat_mode", false);
 
+// Enable TLS 1.3 hello downgrade sentinel?
+// One of the key protections offered by TLS 1.3 is preventing protocol downgrades
+// as part of the initial handshake.
+// Some domains, middleware and transparent routers may try to downgrade connections
+// this way (which is a bad thing!). To allow users to connect anyway this
+// check can be disabled here. Default is for the sentinel to be enabled, preventing
+// bad downgrades of the protocol version.
+pref("security.tls.hello_downgrade_check", true);
+
 // 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);
diff --git a/security/manager/ssl/nsNSSComponent.cpp b/security/manager/ssl/nsNSSComponent.cpp
index 57041d0f26..03a7fd7d46 100644
--- a/security/manager/ssl/nsNSSComponent.cpp
+++ b/security/manager/ssl/nsNSSComponent.cpp
@@ -1444,6 +1444,7 @@ static const bool NPN_ENABLED_DEFAULT = true;
 static const bool ALPN_ENABLED_DEFAULT = false;
 static const bool ENABLED_0RTT_DATA_DEFAULT = false;
 static const bool TLS13_COMPAT_MODE_DEFAULT = false;
+static const bool HELLO_DOWNGRADE_CHECK_DEFAULT = true;
 
 static void
 ConfigureTLSSessionIdentifiers()
@@ -1857,7 +1858,7 @@ nsNSSComponent::InitializeNSS()
   SSL_OptionSetDefault(SSL_ENABLE_RENEGOTIATION, SSL_RENEGOTIATE_REQUIRES_XTN);
 
   SSL_OptionSetDefault(SSL_ENABLE_EXTENDED_MASTER_SECRET, true);
-
+  
   SSL_OptionSetDefault(SSL_ENABLE_FALSE_START,
                        Preferences::GetBool("security.ssl.enable_false_start",
                                             FALSE_START_ENABLED_DEFAULT));
@@ -1881,6 +1882,13 @@ nsNSSComponent::InitializeNSS()
   SSL_OptionSetDefault(SSL_ENABLE_TLS13_COMPAT_MODE,
                        Preferences::GetBool("security.ssl.enable_tls13_compat_mode",
                                             TLS13_COMPAT_MODE_DEFAULT));  
+
+  // Set TLS 1.3 hello downgrade sentinel?
+  bool enableDowngradeCheck =
+    Preferences::GetBool("security.tls.hello_downgrade_check",
+                         HELLO_DOWNGRADE_CHECK_DEFAULT);
+  SSL_OptionSetDefault(SSL_ENABLE_HELLO_DOWNGRADE_CHECK, enableDowngradeCheck);
+
   
   if (NS_FAILED(InitializeCipherSuite())) {
 
@@ -2044,6 +2052,11 @@ nsNSSComponent::Observe(nsISupports* aSubject, const char* aTopic,
     if (prefName.EqualsLiteral("security.tls.version.min") ||
         prefName.EqualsLiteral("security.tls.version.max")) {
       (void) setEnabledTLSVersions();
+    } else if (prefName.EqualsLiteral("security.tls.hello_downgrade_check")) {
+      bool enableDowngradeCheck =
+        Preferences::GetBool("security.tls.hello_downgrade_check",
+                             HELLO_DOWNGRADE_CHECK_DEFAULT);
+      SSL_OptionSetDefault(SSL_ENABLE_HELLO_DOWNGRADE_CHECK, enableDowngradeCheck);
     } else if (prefName.EqualsLiteral("security.ssl.require_safe_negotiation")) {
       bool requireSafeNegotiation =
         Preferences::GetBool("security.ssl.require_safe_negotiation",