Examine source code of Tab List Button

{
  "applications": {
    "gecko": {
      "id": "tab-list-single@codefisher.org",
      "strict_min_version": "42.0"
    }
  },
  "author": "Michael Buckley",
  "browser_action": {
    "browser_style": true,
    "default_icon": {
      "16": "icons/16-tabs.png",
      "24": "icons/24-tabs.png",
      "32": "icons/32-tabs.png",
      "48": "icons/48-tabs.png"
    },
    "default_popup": "popup/panel.html",
    "default_title": "__MSG_tabListLabel__"
  },
  "default_locale": "en_US",
  "description": "When you have lots of tabs open, it is good to have another way of selecting one.",
  "homepage_url": "https://codefisher.org/toolbar_button/",
  "icons": {
    "16": "icons/16-tabs.png",
    "24": "icons/24-tabs.png",
    "32": "icons/32-tabs.png",
    "48": "icons/48-tabs.png"
  },
  "manifest_version": 2,
  "name": "Tab List Button",
  "permissions": [
    "tabs"
  ],
  "version": "1.4resigned1"
}