Examine source code of TabCount

{
  "manifest_version": 2,
  "name": "TabCount",
  "version": "1.0",
  "description": "Adds a red border to all webpages matching mozilla.org",
  "icons": {
    "48": "icons/border-48.png"
  },
  "browser_action": {
    "default_icon": "icons/border-48.png",
    "default_title": "Tab Count"
  },
  "background": {
    "scripts": [
      "tabcount.js"
    ]
  }
}