Examine source code of User agent override

{
  "manifest_version": 2,
  "version": "1.0",
  "name": "User agent override",
  "description": "Gives the ability to set custom user agent string",
  "author": "Nikola Glavina",
  "browser_specific_settings": {
    "gecko": {
      "id": "useragentoverride@glavina.nikola"
    }
  },
  "background": {
    "scripts": [
      "intercept.js"
    ]
  },
  "permissions": [
    "webRequest",
    "<all_urls>",
    "webRequestBlocking",
    "storage"
  ],
  "options_ui": {
    "page": "options.html",
    "browser_style": true
  }
}