Examine source code of One two flash

{
  "manifest_version": 2,
  "name": "One two flash",
  "version": "1.1",
  "description": "Firefox tab counter that lets you jump to tabs with numbers like FLASHHHHH!",
  "background": {
    "scripts": [
      "background.js"
    ]
  },
  "permissions": [
    "tabs",
    "*://*/*"
  ],
  "browser_action": {
    "default_popup": "popup/popup.html",
    "default_icon": "icon.png"
  },
  "content_scripts": [
    {
      "matches": [
        "<all_urls>"
      ],
      "js": [
        "content.js"
      ],
      "run_at": "document_idle"
    }
  ],
  "browser_specific_settings": {
    "gecko": {
      "id": "{9e5dfafe-312a-491b-9251-93894b7bf7a0}"
    }
  }
}