Examine source code of Better letter

{
  "manifest_version": 2,
  "name": "Better letter",
  "description": "Write letters easier.",
  "version": "3.0",
  "icons": {
    "16": "./icons/icon-16.png",
    "32": "./icons/icon-32.png",
    "48": "./icons/icon-48.png",
    "128": "./icons/icon-128.png"
  },
  "content_scripts": [
    {
      "matches": [
        "<all_urls>"
      ],
      "run_at": "document_idle",
      "all_frames": false,
      "js": [
        "content-script.js"
      ]
    }
  ],
  "web_accessible_resources": [
    "content-script.css",
    "chunk-vendors.js",
    "app.js"
  ],
  "browser_specific_settings": {
    "gecko": {
      "id": "pasyukgeorgy@gmail.com",
      "strict_min_version": "42.0"
    }
  },
  "permissions": [
    "storage",
    "<all_urls>"
  ],
  "browser_action": {
    "default_popup": "popup/popup.html"
  }
}