Examine source code of Characters Counter

{
  "manifest_version": 2,
  "name": "Character Counter",
  "version": "0.1.2resigned1",
  "description": "Display the number of characters selected, inspired by 45to75 (Chrome) - https://chrome.google.com/webstore/detail/45to75/efmppndinjbljeellfdkpghgblenbcdd",
  "icons": {
    "16": "icons/icon-16.png",
    "32": "icons/icon-32.png",
    "64": "icons/icon-64.png",
    "128": "icons/icon-128.png"
  },
  "permissions": [
    "contextMenus",
    "activeTab"
  ],
  "background": {
    "scripts": [
      "character_counter_menu.js"
    ]
  },
  "content_scripts": [
    {
      "matches": [
        "<all_urls>"
      ],
      "js": [
        "character_counter.js"
      ]
    }
  ],
  "browser_specific_settings": {
    "gecko": {
      "id": "@characterscounter"
    }
  }
}