Examine source code of imgflipper

{
  "manifest_version": 2,
  "name": "imgflipper",
  "version": "1.0",
  "description": "download memes from imgflip without watermarks",
  "permissions": [
    "activeTab"
  ],
  "content_scripts": [
    {
      "matches": [
        "*://imgflip.com/*"
      ],
      "js": [
        "script.js"
      ]
    }
  ],
  "browser_specific_settings": {
    "gecko": {
      "id": "{fbb6886f-e47f-43e2-8bcc-63c5ab41bd2f}"
    }
  }
}