basementhud/overlay/usr/lib64/firefox/distribution/policies.json

{
    "policies": {
        "BlockAboutConfig": true,
        "Extensions": {
            "Install": [
                "https://addons.mozilla.org/firefox/downloads/file/1105247/auto_fullscreen-1.0.0.2-fx.xpi"
            ]
        }
    }
}