diff --git a/1.3-Implementation.md b/1.3-Implementation.md
index 00ae0e7..45c1276 100644
--- a/1.3-Implementation.md
+++ b/1.3-Implementation.md
@@ -22,13 +22,15 @@
   * BUT... and this may come as a nasty surprise to some, we DO delete history - download, form and browsing histories. If you want to keep them, comment out those preferences
     * Note: Since FF55 we disabled search & form history (0860)
 
-:exclamation: VERY IMPORTANT: Cookies and Extensions
+:exclamation: VERY IMPORTANT: Cookies, Extensions & First Party Isolation
 
 - All cookies are disabled (2701) by default
   * Cookies control access to local storage and IndexedDB
   * Extensions (not Legacy Extensions) have more limited options for persistent storage, and many use IndexedDB
   * If you block all cookies by default, then you will need to allow a cookie exception for some Extensions, including uBlock Origin, uMatrix, Stylus, and Violentmonkey
   * See [Setting Extension Permission Exceptions](https://github.com/ghacksuserjs/ghacks-user.js/wiki/1.7-Setting-Extension-Permission-Exceptions)
+- First Party Isolation (4001) is enabled by default
+   * Cookie extensions currently cannot control persistent storage with FPI Origin Attributes <sup>[1381197](https://bugzilla.mozilla.org/show_bug.cgi?id=1381197)</sup>
 
 :exclamation: REALLY IMPORTANT: privacy.resistFingerprinting (RFP)