Examine source code of Sentence Count

{
  "manifest_version": 2,
  "name": "Sentence Counter",
  "version": "1.0",
  "description": "Count sentences in selected text",
  "icons": {
    "48": "sentencecounter48.png",
    "96": "sentencecounter96.png"
  },
  "permissions": [
    "activeTab"
  ],
  "browser_action": {
    "default_popup": "popup/popup.html",
    "default_icon": {
      "48": "sentencecounter48.png",
      "96": "sentencecounter96.png"
    }
  },
  "content_scripts": [
    {
      "matches": [
        "<all_urls>"
      ],
      "js": [
        "content.js"
      ]
    }
  ],
  "background": {
    "scripts": [
      "background.js"
    ]
  },
  "browser_specific_settings": {
    "gecko": {
      "id": "{913b7708-e3a7-45e6-8202-91f6bb7cdc1d}"
    }
  }
}