Examine source code of Character Count

{
  "manifest_version": 2,
  "name": "Character Count",
  "version": "1.1.2",
  "description": "Character Count Extension for Firefox - Word & Character Counter.",
  "permissions": [
    "storage",
    "activeTab",
    "tabs",
    "<all_urls>"
  ],
  "browser_action": {
    "default_popup": "popup.html",
    "default_icon": {
      "16": "icons/icon16.png",
      "48": "icons/icon48.png",
      "128": "icons/icon128.png"
    }
  },
  "background": {
    "scripts": [
      "background.js"
    ],
    "persistent": false
  },
  "icons": {
    "16": "icons/icon16.png",
    "48": "icons/icon48.png",
    "128": "icons/icon128.png"
  },
  "content_scripts": [
    {
      "matches": [
        "<all_urls>"
      ],
      "js": [
        "content.js"
      ],
      "run_at": "document_end"
    }
  ],
  "content_security_policy": "script-src 'self'; object-src 'none'",
  "applications": {
    "gecko": {
      "id": "character-count@character-count.net",
      "strict_min_version": "58.0"
    }
  }
}