Examine source code of SilentGIF

{
  "manifest_version": 2,
  "name": "SilentGIF",
  "version": "1.0",
  "description": "Replaces autoplaying GIFs with still preview. Plays only on hover.",
  "permissions": [
    "activeTab"
  ],
  "content_scripts": [
    {
      "matches": [
        "<all_urls>"
      ],
      "js": [
        "content.js"
      ],
      "run_at": "document_idle"
    }
  ],
  "icons": {
    "48": "icon.png"
  },
  "browser_specific_settings": {
    "gecko": {
      "id": "{67a72385-c25f-4e71-b49e-0ffec2bac487}"
    }
  }
}