Examine source code of Reading Ruler Plug in

{
  "manifest_version": 2,
  "name": "Reading Ruler",
  "version": "1.0",
  "description": "Adds a reading ruler to the user's screen allowing easier readingn of word dense pages.",
  "icons": {
    "48": "icons/Ruler-48.png",
    "96": "icons/Ruler-96.png"
  },
  "permissions": [
    "activeTab"
  ],
  "browser_action": {
    "default_icon": "icons/Ruler-32.png",
    "default_title": "Reading Ruler",
    "default_popup": "popup/ruler_settings.html"
  },
  "web_accessible_resources": [
    "/content_scripts/readingRuler.js",
    "/content_scripts/rulerStyles.css"
  ],
  "content_scripts": [
    {
      "matches": [
        "<all_urls>",
        "file:///*"
      ],
      "js": [
        "content_scripts/readingRuler.js"
      ]
    }
  ],
  "browser_specific_settings": {
    "gecko": {
      "id": "{a544a214-f9fa-4e23-865e-e7ddd3c1b600}"
    }
  }
}