Examine source code of Keyword Density Checker

{
  "manifest_version": 2,
  "name": "Keyword Density Checker",
  "version": "1.0",
  "description": "A Mozilla add-on that checks the keyword density of a web page.",
  "icons": {
    "48": "icon.png"
  },
  "permissions": [
    "activeTab"
  ],
  "browser_action": {
    "default_icon": {
      "48": "icon.png"
    },
    "default_popup": "popup.html"
  },
  "content_scripts": [
    {
      "matches": [
        "<all_urls>"
      ],
      "js": [
        "contentScript.js"
      ]
    }
  ],
  "web_accessible_resources": [
    "popup.html",
    "popup.css",
    "popup.js"
  ],
  "browser_specific_settings": {
    "gecko": {
      "id": "{083215ee-fab5-4f7e-a16d-0d52f1595ebc}"
    }
  }
}