diff --git a/conf/firefox-forbidden-policies.js b/conf/firefox-forbidden-policies.js
index f8c4dcd4..ffdad897 100644
--- a/conf/firefox-forbidden-policies.js
+++ b/conf/firefox-forbidden-policies.js
@@ -63,6 +63,10 @@ lockPref("privacy.resistFingerprinting.letterboxing", true);
 //pref("privacy.resistFingerprinting.pbmode", false);
 clearPref("privacy.resistFingerprinting.pbmode");
 
+// Which extension requires container tabs? My policy and this file. This
+// particular option is so forbidden, it cannot be set either in the policy
+// or autoconfig.js
+lockPref("privacy.userContext.extension", "");
 // Enable containers without extensions (although those are forced by the
 // policy. TODO: Remove these as they are in the policy, but disallowed by
 // current ESR
diff --git a/etc/firefox/policies/policies.json b/etc/firefox/policies/policies.json
index f7426baa..e7e02cde 100644
--- a/etc/firefox/policies/policies.json
+++ b/etc/firefox/policies/policies.json
@@ -668,6 +668,12 @@
         "Type": "boolean",
         "Value": true
       },
+      "privacy.userContext.extension": {
+        "Comment": "Displays in settings which extension requires container tabs. None. it's this policy. Preference not allowed for stability reasons.",
+        "Status": "locked",
+        "Type": "string",
+        "Value": ""
+      },
       "privacy.userContext.ui.enabled": {
         "Comment": "Tab containers UI without extensions",
         "Status": "locked",