FirefoxでのWebextensionのインストールに失敗します

about:debuggingから一時的なアドオンを読み込むをクリックし、manifest.jsonを選択するとThere was an error during installation: Extension is invalidとだけ表示されます

どうすれば良いのでしょうか?

manifest.json


{
    "manifest_version": 2,
    "name": "Beastify",
    "version": 1.0,

    "description": "add a browser action icon to the toolbar. Click the button to choose a beast. The active tab's body content is then replaced with a picture of the choosen beast.",
    "icons": {
        "48": "icons/beasts-48.png"
    },

    "permissions": [
        "activeTab"
    ],

    "browser_action": {
        "default_icon": "icons/beasts-32.png",
        "default_title": "Beastify",
        "default_popup": "popup/choose_beast.html"
    },

    "web_accessible_resources": [
        "beasts/frog.jpg",
        "beasts/turtle.jpg",
        "beasts/snake.jpg"
    ]
}