Add FirefoxExtensions.json

copy template from mozilla github, just works pasted into GPO:
Computer Configuration > Policies > Administrative Templates > Mozilla > Firefox >Extensions > Extension Management

FirefoxExtensions.json

@@ -0,0 +1,16 @@
+{
+  "*": {
+    "blocked_install_message": "Custom error message.",
+    "install_sources": ["https://yourwebsite.com/*"],
+    "installation_mode": "blocked",
+    "allowed_types": ["extension"]
+  },
+  "uBlock0@raymondhill.net": {
+    "installation_mode": "force_installed",
+    "install_url": "https://addons.mozilla.org/firefox/downloads/latest/ublock-origin/latest.xpi"
+  },
+  "https-everywhere@eff.org": {
+    "installation_mode": "allowed",
+    "updates_disabled": false
+  }
+}