Examine source code of Mute Tab

{
  "description": "Mute Tab add-on.",
  "manifest_version": 2,
  "name": "Mute Tab",
  "version": "0.2.9resigned1",
  "applications": {
    "gecko": {
      "id": "{d3d46ca0-999d-11da-a72b-0800200c9a66}"
    }
  },
  "permissions": [
    "menus"
  ],
  "background": {
    "scripts": [
      "background.js"
    ]
  }
}