Examine source code of Tab Timer i

{
  "manifest_version": 2,
  "name": "Tab Timer",
  "version": "1.0",
  "description": "Reminds you to take a break after spending too much time on a tab.",
  "icons": {
    "48": "icon.png"
  },
  "background": {
    "scripts": [
      "background.js"
    ]
  },
  "permissions": [
    "tabs",
    "notifications"
  ],
  "browser_specific_settings": {
    "gecko": {
      "id": "{88b0a4fc-df77-4dfb-a8c4-2b80a1c94e22}"
    }
  }
}