diff --git a/dom/security/nsContentSecurityManager.cpp b/dom/security/nsContentSecurityManager.cpp
index 84f97c041..9006910f1 100644
--- a/dom/security/nsContentSecurityManager.cpp
+++ b/dom/security/nsContentSecurityManager.cpp
@@ -102,6 +102,9 @@ nsContentSecurityManager::CheckFTPSubresourceLoad(nsIChannel* aChannel)
   // We dissallow using FTP resources as a subresource everywhere.
   // The only valid way to use FTP resources is loading it as
   // a top level document.
+  if (!mozilla::net::nsIOService::BlockFTPSubresources()) {
+    return NS_OK;
+  }
 
   nsCOMPtr<nsILoadInfo> loadInfo = aChannel->GetLoadInfo();
   if (!loadInfo) {
diff --git a/modules/libpref/init/all.js b/modules/libpref/init/all.js
index 837a9eb02..e8bd211cb 100644
--- a/modules/libpref/init/all.js
+++ b/modules/libpref/init/all.js
@@ -5575,6 +5575,9 @@ pref ("security.mixed_content.hsts_priming_request_timeout", 3000);
 // URL-Bar will not be blocked when flipping this pref.
 pref("security.data_uri.block_toplevel_data_uri_navigations", true);
 
+// If true, all FTP subresource loads will be blocked.
+pref("security.block_ftp_subresources", true);
+
 // Disable Storage api in release builds.
 #ifdef NIGHTLY_BUILD
 pref("dom.storageManager.enabled", true);
diff --git a/netwerk/base/nsIOService.cpp b/netwerk/base/nsIOService.cpp
index 0244807f3..3b7d28264 100644
--- a/netwerk/base/nsIOService.cpp
+++ b/netwerk/base/nsIOService.cpp
@@ -188,6 +188,7 @@ uint32_t   nsIOService::gDefaultSegmentCount = 24;
 bool nsIOService::sTelemetryEnabled = false;
 
 bool nsIOService::sBlockToplevelDataUriNavigations = false;
+bool nsIOService::sBlockFTPSubresources = false;
 
 ////////////////////////////////////////////////////////////////////////////////
 
@@ -269,6 +270,8 @@ nsIOService::Init()
     Preferences::AddBoolVarCache(&sTelemetryEnabled, "toolkit.telemetry.enabled", false);
     Preferences::AddBoolVarCache(&sBlockToplevelDataUriNavigations,
                                  "security.data_uri.block_toplevel_data_uri_navigations", false);
+    Preferences::AddBoolVarCache(&sBlockFTPSubresources,
+                                 "security.block_ftp_subresources", true);
     Preferences::AddBoolVarCache(&mOfflineMirrorsConnectivity, OFFLINE_MIRRORS_CONNECTIVITY, true);
 
     gIOService = this;
@@ -1902,5 +1905,11 @@ nsIOService::BlockToplevelDataUriNavigations()
   return sBlockToplevelDataUriNavigations;
 }
 
+/*static*/ bool
+nsIOService::BlockFTPSubresources()
+{
+  return sBlockFTPSubresources;
+}
+
 } // namespace net
 } // namespace mozilla
diff --git a/netwerk/base/nsIOService.h b/netwerk/base/nsIOService.h
index e592c4d1c..a7063d416 100644
--- a/netwerk/base/nsIOService.h
+++ b/netwerk/base/nsIOService.h
@@ -97,6 +97,8 @@ public:
 
     static bool BlockToplevelDataUriNavigations();
 
+    static bool BlockFTPSubresources();
+
     // Used to trigger a recheck of the captive portal status
     nsresult RecheckCaptivePortal();
 private:
@@ -180,6 +182,8 @@ private:
 
     static bool                          sBlockToplevelDataUriNavigations;
 
+    static bool                          sBlockFTPSubresources;
+
     // These timestamps are needed for collecting telemetry on PR_Connect,
     // PR_ConnectContinue and PR_Close blocking time.  If we spend very long
     // time in any of these functions we want to know if and what network