Examine source code of Remove Visited Link

{
  "applications": {
    "gecko": {
      "id": "jid1-8e4xN5rmRmX62g@jetpack",
      "strict_min_version": "50.0"
    }
  },
  "manifest_version": 2,
  "name": "Remove Visited Link",
  "version": "0.5resigned1",
  "description": "Change a visited link to unvisited",
  "homepage_url": "https://addons.mozilla.org/ja/firefox/addon/remove-visited-link/",
  "default_locale": "en",
  "background": {
    "scripts": [
      "background.js"
    ]
  },
  "content_scripts": [
    {
      "matches": [
        "<all_urls>"
      ],
      "js": [
        "content.js"
      ]
    }
  ],
  "permissions": [
    "<all_urls>",
    "contextMenus",
    "history"
  ]
}