Examine source code of Block Mailtrack

{
  "manifest_version": 2,
  "name": "Block Mailtrack",
  "version": "1.1",
  "description": "Block tracking pixel from mailtrack.io (for Gmail).",
  "homepage_url": "https://github.com/gioxx/block-mailtrack",
  "browser_specific_settings": {
    "gecko": {
      "id": "block-mailtrack@noads.it"
    }
  },
  "icons": {
    "16": "icons/icon16.png",
    "32": "icons/icon32.png",
    "48": "icons/icon48.png",
    "64": "icons/icon64.png",
    "128": "icons/icon128.png"
  },
  "permissions": [
    "webRequest",
    "webRequestBlocking",
    "*://*.googleusercontent.com/",
    "*://mail.google.com/*"
  ],
  "background": {
    "scripts": [
      "background.js"
    ]
  },
  "content_scripts": [
    {
      "matches": [
        "*://*.googleusercontent.com/",
        "*://mail.google.com/*"
      ],
      "css": [
        "mtsignature.css"
      ]
    }
  ]
}