In the manifest it has a special "experiment_apis":

    "experiment_apis": {
        "skeleton": {
            "schema": "experiments/skeleton/schema.json",
            "parent": {
                "scopes": [
                    "addon_parent"
                ],
                "script": "experiments/skeleton/api.js",
                "paths": [
                    [
                        "experiments",
                        "skeleton"
                    ]
                ]
            }
        }
    }

Only Mozilla can use these on release versions of Firefox. If you want some more details then try here: https://firefox-source-docs.mozilla.org/toolkit/components/e...