Examine source code of tst-tab-count

{
  "manifest_version": 2,
  "name": "tst-tab-count",
  "description": "Show tab count in 'new tab' button",
  "version": "1.0.0",
  "icons": {
    "64": "icons/icon.png"
  },
  "background": {
    "scripts": [
      "tabCount.js"
    ]
  },
  "page_action": {
    "default_icon": {
      "64": "icons/icon.png"
    },
    "default_popup": "pageAction/index.html",
    "default_title": "tst-tab-count"
  },
  "browser_specific_settings": {
    "gecko": {
      "id": "{1b63e36d-f064-49bc-ad84-8ee8c43ed9f2}"
    }
  }
}