Examine source code of Sound Tabs

{
  "manifest_version": 2,
  "name": "Sound Tabs",
  "version": "1.0",
  "description": "Plays drum sounds for tab events: switch tab = hit, close = cymbal crash, create = kick.",
  "icons": {
    "48": "icon-48.png",
    "128": "icon-128.png"
  },
  "background": {
    "scripts": [
      "background.js"
    ],
    "persistent": true
  },
  "permissions": [
    "tabs"
  ],
  "applications": {
    "gecko": {
      "id": "soundtabs@example.com"
    }
  }
}