Examine source code of Mouse Piano

{
  "manifest_version": 2,
  "name": "Mouse Piano",
  "version": "1.0",
  "description": "Play pitches by moving your mouse across the page — X = pitch, Y = volume/timbre. Toggle on/off with the floating button.",
  "icons": {
    "48": "icon-48.png",
    "128": "icon-128.png"
  },
  "content_scripts": [
    {
      "matches": [
        "<all_urls>"
      ],
      "js": [
        "content_script.js"
      ],
      "run_at": "document_idle"
    }
  ],
  "permissions": [
    "<all_urls>"
  ],
  "browser_specific_settings": {
    "gecko": {
      "id": "{6a211310-8ed5-49b2-b25c-081e8041de48}"
    }
  }
}