Examine source code of User Agent Masquerade

{
  "manifest_version": 2,
  "name": "User Agent Masquerade",
  "version": "0.3.1",
  "description": "Adds a button to the toolbar to switch the User-Agent header.",
  "icons": {
    "48": "icon.svg"
  },
  "permissions": [
    "storage",
    "webRequest",
    "webRequestBlocking",
    "<all_urls>"
  ],
  "background": {
    "scripts": [
      "background.js"
    ]
  },
  "options_ui": {
    "page": "options.html",
    "browser_style": true
  },
  "browser_action": {
    "browser_style": true,
    "default_icon": "default.svg",
    "default_title": "User Agent Masquerade",
    "default_popup": "popup.html"
  },
  "browser_specific_settings": {
    "gecko": {
      "id": "{08c422b9-f476-4b0f-a66c-4b7559087498}"
    }
  }
}