Examine source code of Instant Image Compare

{
  "manifest_version": 2,
  "name": "Instant Image Compare",
  "version": "1.0.1",
  "description": "Shift-hover an image to open a side-by-side comparison window.",
  "permissions": [
    "tabs"
  ],
  "background": {
    "scripts": [
      "background.js"
    ]
  },
  "content_scripts": [
    {
      "matches": [
        "<all_urls>"
      ],
      "js": [
        "content.js"
      ],
      "run_at": "document_idle"
    }
  ],
  "browser_specific_settings": {
    "gecko": {
      "id": "instant-image-compare-fixed@example.com",
      "data_collection_permissions": {
        "required": [
          "none"
        ],
        "collects": false
      }
    }
  }
}