Examine source code of Font Adjuster

{
  "name": "Font Adjuster",
  "version": "0.0.4",
  "description": "Increase or decrease font size.",
  "manifest_version": 2,
  "icons": {
    "16": "assets/images/16.png",
    "32": "assets/images/32.png",
    "48": "assets/images/48.png",
    "128": "assets/images/128.png"
  },
  "browser_action": {
    "default_popup": "popup.html",
    "browser_style": false
  },
  "content_scripts": [
    {
      "all_frames": true,
      "css": [],
      "js": [
        "assets/js/fontsize.js"
      ],
      "matches": [
        "<all_urls>"
      ],
      "run_at": "document_start"
    }
  ],
  "browser_specific_settings": {
    "gecko": {
      "id": "{30a7314c-2cbe-4f89-b6ae-e4531e711d1d}"
    }
  }
}