Examine source code of screenkey

{
  "manifest_version": 2,
  "name": "addon_screen_key",
  "author": "aloisleclet",
  "version": "1.0.0",
  "description": "Show your key pressed during your screen share & streams.",
  "homepage_url": "https://github.com/aloisleclet/addon_screen_key",
  "icons": {
    "16": "./icons/on.png",
    "128": "./icons/on.png"
  },
  "browser_action": {
    "default_icon": "./icons/off.png",
    "default_title": "disabled"
  },
  "content_scripts": [
    {
      "matches": [
        "*://*/*"
      ],
      "js": [
        "content.js"
      ]
    }
  ],
  "background": {
    "scripts": [
      "background.js"
    ]
  },
  "permissions": [
    "tabs",
    "activeTab",
    "*://*/*"
  ]
}