diff --git a/system/firefox.nix b/system/firefox.nix
index 5094ae05..6b5d9ad7 100644
--- a/system/firefox.nix
+++ b/system/firefox.nix
@@ -5,7 +5,10 @@
     enable = true;
     languagePacks = [ "en-GB" ];
     policies = {
-      Cookies = { Behaviour = "reject-tracker-and-partition-foreign"; };
+      Cookies = {
+        Behavior = "reject-tracker-and-partition-foreign";
+        Locked = true;
+      };
       DisableAppUpdate = true;
       DisableBuiltinPDFViewer = true;
       DisableFirefoxAccounts = true;