Examine source code of DefaultMono

{
  "manifest_version": 2,
  "name": "DefaultMono",
  "version": "2.3",
  "description": "Reset monospace fonts to the default one.",
  "icons": {
    "48": "icons/icon-48.png"
  },
  "content_scripts": [
    {
      "matches": [
        "<all_urls>"
      ],
      "js": [
        "defaultmono.js"
      ]
    }
  ],
  "browser_specific_settings": {
    "gecko": {
      "id": "{a9c91f54-5028-41cf-9159-980bd947bc4d}"
    }
  }
}