Examine source code of Tab Timer it

{
  "manifest_version": 2,
  "name": "Tab Timer",
  "version": "1.0",
  "description": "Set a timer to auto-close the current tab.",
  "permissions": [
    "tabs"
  ],
  "browser_action": {
    "default_icon": "icon.png",
    "default_title": "Tab Timer",
    "default_popup": "popup.html"
  },
  "icons": {
    "48": "icon.png"
  },
  "browser_specific_settings": {
    "gecko": {
      "id": "{853f9702-59cc-4dac-9bcb-c3ba695eed0e}"
    }
  }
}