Examine source code of myonlyextension1291

{
  "description": "Adds browser action icon to toolbar to choose user agent string from popup menu. See https://developer.mozilla.org/en-US/Add-ons/WebExtensions/Examples#user-agent-rewriter",
  "manifest_version": 2,
  "name": "user-agent-rewriter",
  "version": "1.0",
  "homepage_url": "https://github.com/mdn/webextensions-examples/tree/master/user-agent-rewriter",
  "icons": {
    "48": "icons/person-48.png"
  },
  "permissions": [
    "webRequest",
    "webRequestBlocking",
    "<all_urls>"
  ],
  "background": {
    "scripts": [
      "background.js"
    ]
  },
  "content_scripts": [
    {
      "run_at": "document_start",
      "js": [
        "contentScript.js"
      ],
      "matches": [
        "<all_urls>"
      ],
      "all_frames": true
    }
  ],
  "browser_action": {
    "default_icon": "icons/person-32.png",
    "default_title": "Choose a user agent"
  }
}