Examine source code of Last Visited Toolip

{
  "manifest_version": 2,
  "name": "Last Visited Toolip",
  "version": "0.1.5resigned1",
  "description": "Shows last visit time in link toolips.",
  "icons": {
    "48": "icons/icon-48.png",
    "96": "icons/icon-96.png"
  },
  "author": "Rotem Dan",
  "homepage_url": "https://github.com/rotemdan/LastVisitedTooltip",
  "background": {
    "scripts": [
      "browser-polyfill.min.js",
      "common.js",
      "background.js"
    ]
  },
  "content_scripts": [
    {
      "matches": [
        "<all_urls>"
      ],
      "js": [
        "browser-polyfill.min.js",
        "common.js",
        "content.js"
      ],
      "run_at": "document_start",
      "all_frames": true,
      "match_about_blank": false
    }
  ],
  "permissions": [
    "history"
  ],
  "browser_specific_settings": {
    "gecko": {
      "id": "{16615320-157b-4e6b-b767-c7f5210792bb}"
    }
  }
}