Examine source code of Close tabs by...

{
  "manifest_version": 2,
  "name": "Close tabs by...",
  "version": "0.1.4",
  "description": "Close tabs with same domain or containing a string in the title",
  "applications": {
    "gecko": {
      "id": "close-tabs-by@bekh-ivanov.me",
      "strict_min_version": "45.0"
    }
  },
  "browser_action": {
    "default_icon": "icon.svg",
    "default_popup": "popup/input.html"
  },
  "icons": {
    "48": "icon.svg",
    "96": "icon.svg"
  },
  "permissions": [
    "menus",
    "contextMenus",
    "tabs"
  ],
  "background": {
    "scripts": [
      "background-script.js"
    ]
  }
}