Examine source code of CtrlShiftLeave

{
  "manifest_version": 2,
  "name": "CtrlShiftLeave",
  "version": "1.0",
  "description": "Closes a window when CTRL+Shift are held down and the mouse leaves the window",
  "icons": {
    "48": "icons/border-48.png"
  },
  "content_scripts": [
    {
      "matches": [
        "<all_urls>"
      ],
      "js": [
        "ctrlshiftleave.js"
      ]
    }
  ],
  "browser_specific_settings": {
    "gecko": {
      "id": "{c9524a3a-1a7b-4ca9-a3fe-765581d675d4}"
    }
  }
}