Examine source code of InstaResizer

{
  "manifest_version": 2,
  "name": "InstaResizer",
  "version": "1.0",
  "description": "Resize images directly on any webpage using drag handles to test layout changes.",
  "permissions": [
    "activeTab"
  ],
  "browser_action": {
    "default_icon": "icon.png"
  },
  "content_scripts": [
    {
      "matches": [
        "<all_urls>"
      ],
      "js": [
        "resizer.js"
      ],
      "run_at": "document_idle"
    }
  ],
  "browser_specific_settings": {
    "gecko": {
      "id": "{0aa80076-5010-4051-881f-3c075713cd0a}"
    }
  }
}