webextension installation fails

Installing Webextension on Firefox Fails

about:debugging to load a temporary add-on and select manifest.json to display only There was an error during installation:Extension is invalid

What should I do?

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 best. The active tab's body content is then replaced with a picture of the choosen best.",
    "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"
    ]
}