Examine source code of Speech Timer

{
  "manifest_version": 2,
  "name": "Speech Timer",
  "version": "1.0.1",
  "description": "",
  "icons": {
    "48": "icons/icon48.png",
    "96": "icons/icon96.png"
  },
  "permissions": [
    "activeTab"
  ],
  "browser_action": {
    "default_icon": "icons/icon32.png",
    "default_title": "Speech Timer"
  },
  "applications": {
    "gecko": {
      "id": "speech-timer@bravikov.pro",
      "strict_min_version": "55.0"
    }
  },
  "background": {
    "scripts": [
      "background-script.js"
    ]
  },
  "content_scripts": [
    {
      "matches": [
        "<all_urls>"
      ],
      "js": [
        "speech-timer.js"
      ]
    }
  ]
}