Examine source code of Ew, Mail!

{
  "manifest_version": 2,
  "name": "Ew, Mail!",
  "version": "1.2",
  "description": "Ew, Mail! allows you to easily use a temp-mail.org mail when registering.",
  "homepage_url": "https://github.com/vantezzen/ewmail",
  "icons": {
    "48": "icons/icon-48.png"
  },
  "permissions": [
    "activeTab",
    "tabs",
    "https://temp-mail.org/",
    "contextMenus"
  ],
  "background": {
    "scripts": [
      "background-scripts.js"
    ]
  },
  "content_scripts": [
    {
      "all_frames": true,
      "js": [
        "content.js"
      ],
      "matches": [
        "http://*/*",
        "https://*/*",
        "file:///*"
      ],
      "run_at": "document_start"
    }
  ]
}