Examine source code of Focus Timer it

{
  "manifest_version": 2,
  "name": "Focus Timer",
  "version": "1.0",
  "description": "Pomodoro-style focus timer.",
  "icons": {
    "48": "icon.png"
  },
  "browser_action": {
    "default_popup": "popup.html",
    "default_icon": "icon.png"
  },
  "permissions": [],
  "browser_specific_settings": {
    "gecko": {
      "id": "{f50743f0-1a26-412a-9792-4fa47909dde4}"
    }
  }
}