policies.json file for doh and extension restrictions

I want to implement child protection on single machines: family filter and custom blocklisting of websites (social media prevalently)

–

System:

Windows 11 Home 64 23h2

browser Firefox version 142.0.1 (64)

the user will be a standard user without admin privileges

I try to implement it from an admin account

–

Two measures have to be deployed:

doh with cleanbrowsing family filter. standard users restricted from tampering with it
forceinstall of the extension leechblock ng and restrict standard users from tampering with it (if you have a better extension alternative is also welcome, leechblock ng is free has all the functions I need and I can find my way around it)
It doesn’t have to happen through policies it just seemed the simplest method and required also not to change things such as registry keys. It is important that the method is also easily teachable, as we are a small school with very limited resources.

–

I tried to create the policies.json file in C:Program FilesMozilla Firefoxdistribution

{
“policies”: {
“BlockAboutConfig”: true,
“Preferences”: {
“network.trr.mode”: {
“Value”: 3,
“Status”: “locked”
},
“network.trr.uri”: {
“Value”: “https://doh.cleanbrowsing.org/doh/family-filter/”,
“Status”: “locked”
}
},
“ExtensionSettings”: {
“*”: {
“installation_mode”: “blocked”
},
“leechblockng@proginosko.com”: {
“installation_mode”: “force_installed”,
“install_url”: “https://addons.mozilla.org/firefox/downloads/latest/leechblock-ng/latest.xpi”
}
}
}
}

–

This seems to have nailed the doh part but the extension part just won’t work.

–

After creating the policies.json file and closing and reopening of Firefox (no task running in task manager) the extension is not installed and it obviously also keeps me from installing it manually.