Examine source code of Restore closed tab button

{
  "manifest_version": 2,
  "name": "__MSG_extensionName__",
  "version": "1.0.4resigned1",
  "description": "__MSG_extensionDescription__",
  "default_locale": "en",
  "developer": {
    "name": "Krzysztof Waluś",
    "url": "http://blog.krzysiekw.pl"
  },
  "applications": {
    "gecko": {
      "id": "restoretab@krzysiekw.pl",
      "strict_min_version": "56.0"
    }
  },
  "icons": {
    "48": "icons/re-48.png",
    "96": "icons/re-96.png",
    "128": "icons/re-128.png"
  },
  "permissions": [
    "activeTab",
    "contextMenus",
    "sessions",
    "tabs"
  ],
  "background": {
    "scripts": [
      "background.js"
    ]
  },
  "browser_action": {
    "default_icon": {
      "19": "icons/re-19.png",
      "38": "icons/re-38.png",
      "48": "icons/re-48.png",
      "96": "icons/re-96.png"
    },
    "default_title": "__MSG_buttonTitle__"
  }
}