Examine source code of Tab Counter

{
  "manifest_version": 2,
  "name": "Tab Counter",
  "description": "This extension shows you how many tabs you have open at a given time.",
  "version": "0.1",
  "author": "Lucas Meireles",
  "permissions": [
    "tabs",
    "storage"
  ],
  "background": {
    "page": "./html/background.html"
  },
  "browser_action": {
    "default_popup": "./html/popup.html"
  },
  "browser_specific_settings": {
    "gecko": {
      "id": "{da44e4f9-8b67-40c4-8707-383c6ee095f9}"
    }
  }
}